We have heard that if something is left unchanged for long periods
that people tend to develop a blind spot for it and not notice it
anymore.
eg. With Licence (number) plates on cars, we don?t tend to notice them
unless people add something to them, there may be a colour change of
the Licence (number) plates, or people may put a colourful frame
around them and suddenly our eyes gravitate towards what we took no
notice of in the past.  Although the Licence (number) plates were all
around us before, we didn?t ?pay attention? to them until there was a
slight change. (The overall appearance of the original design stayed
the same, though the addition of the frame brought our attention to
them)
 
(It is basically trying to prove that the house you have never noticed
on your street suddenly gets painted blue, you then notice it every
time you drive past it.)

We would like to find official statistics from Government sources or
reputable market researchers/professors etc., not just statistics from
a magazine or newspaper.

We want to prove 2 things: 

a) We want statistical proof that the more something is in front of
your face, the more likely they are not to see it, or not even realize
it is there. The fact that if something is a constant and ?left
completely unchanged? for long periods then people don?t notice it
anymore, even though it may be all around us.

b) We want statistical evidence that if you keep the original design
of something, but add something to it (even only slight changes), that
it becomes more noticeable. People tend to notice it (for the first
time, or again) even though it was right in front of them the whole
time.
 
(If you come across any information on ?what period of time? it takes
people to ?not notice? something between any physical changes, that
would be great also, eg. after 2 months people develop a blind spot
etc.)

Thank you very  much.

Hullo shapai-ga,
 maybe I can start the ball rolling.  Consider the puzzle of new speed
limit signs... have you ever bumbled into an area where the speed
limit has been reduced and not seen the changed sign ?

 Women are much, much, better at noticing things.. such as what's
buried at the back of the 'fridge and lipstick smudges.

I am surprised that there hasn't till now been a response to your
interesting question.  The problem may be the request for "statistical
proof" and not just learned papers on the subject.
I believe the premise is entirely correct, relating to visual memory,
selective visual memory, and some built-in efficiencies in the system,
which I equate to the way I understand (maybe incorrectly) that
digitalized moving images are compacted by only recording changes from
one monitor image to the next.
The example of the blue house could be straight from my experience. 
There is a street that I often drive through, but since in never stop
in it, all I can recall specifically is that I pass a church and
hairdresser's sign and a side street  -
until one non-descript house was indeed painted blue, which now
catches my eye every time, even after several years.
In our everyday work and home environment, all of us have stored image
of things and immediately recognize a missing, additional or moved
item (not that that always makes it easier to find something).
I have read that Australian Aboriginal trackers don't so much see
tracks  - foot or paw prints -  but recognize disturbances in the
terrain, having a well-trained general visual image of what
undisturbed terrain should look like.

I have also read that women are thought to have a better memory for
things  - now presumably genetically coded -  since in the days of
hunters and gatherers, they were the latter and learned to recall
where plants grew and to recognize them.
But this is not entirely true:  once a woman who knew me only with a
beard, greeted me and rode the elevator with me without noticing that
I had shaved it off, only recognizing the fact when someone else later
mentioned it to her.
Similarly, once when I parted my hair on the other side, only one
person noticed immediately.

It's a very interesting subject.  I hope someone adds some professional links.

For part B, the phenomenon you are looking to research not actual
visual memory, but visual perception (I know they sound the same, but
they are very very different.  The phenomenon is generally referred to
in psychological literature as "feature presence" versus "feature
absence."  Specifcally, When you present a letter sheet, such as
XXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXX
XXXXXXXXXXXPXXXX
XXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXX

it is easy to spot the "P", because the feature "roundness" or
"o-ness" is additionally present.  It results in what is commonly
called "parallel popout" as the P seems to pop out and is clearly
visable at first glance.

However, with feature absence:
RRRRRRRRRRRRRRRR
RRRRRRRRRRRRRRRR
RRRPRRRRRRRRRRRR
RRRRRRRRRRRRRRRR
RRRRRRRRRRRRRRRR
RRRRRRRRRRRRRRRR
RRRRRRRRRRRRRRRR
The "p" located near the upper right quandrant is exceedingly harder
to find.  In attempting to find the "P" among "R"s, the process is
generally called "serial search" because you have to painstakingly
search each row or column to find the P.

There is an entire field of psychology devoted to studying these kinds
of visual perception phenomena.  Anything written by a man named JJ
Gibson (scholar.google it) in the realm of visual perception would
make a great citation.  There are even specific time lapse data
available in Gibson's work and others.  They have actually calculated
the amount of time delay in participant responses based on the type of
feature that is absent or present (i.e., contrast, color, etc.).

Now then, for the first part of your question. There is a sort of
"memory" component, but it's generally not talked about in terms of
"memory", but rather is called often called "selective attention." 
There is neurological research on this subject as well as experimental
research, but the whole of it does assert your main point.  Think
about it and really try to intuit this:  if the brain were to
selectively attend to everything constantly, we wouldn't be able to
really focus on anything with any detail.  Generally, neurological
research describes perception as an active cyclical process in which
we don't really "see" anything except for the things which are
changing.  Our "visual memory" only lasts about 2, maybe 3 seconds. 
So, when something changes or is visually stimulating, we look; then
our physical bodies (eyes) interpret the stimulus into 'data' our
brain can understand, and then either the brain is stimulated and so
we take action by look closer (or picking the object up, etc.) or we
do nothing.  The key here is that if we've already looked at someting
with sufficient repetition, it is no longer mentally stimulating (or
necessarily effortful) and so then we don't bother looking at it
closer any more.  Some great neurological research on this subject is
in a paper at the national academy of sciences by Desimone in 1996.

I think that should sufficiently answer your questions.  But to recap
a) selective attention
b) visual perception