There has been much discussion on various tech forums & blogs about CD
& DVD discs degrading over a very short period. I want to archive my
family photos & videos for at least 20 years or more - or at least
until a more stable medium becomes available. Is there any brand(s) of
CD/DVD blanks which really is stable for a very long period of time
(over 20 years)? Or has this technology been around for such a
relatively short time that - forgetting inane marketing hype - no one
really knows yet?

I think the true answer is that the technology is so new that nobody
knows.  I have some name-brand CDs that I recorded about 15 years ago
that still work fine.  I have been using the cheap "off-brand" CDs
lately, and so far have not had any trouble with CDs "going bad", but
they are only a few years old.

I have also heard that DVDs may be less stable than CDs, but I have
not seen any actual studies on the issue.

The most important thing for long-term storage is probably to keep
them away from heat and light.  And whatever storage method you use,
make an extra copy, test them periodically, and copy to fresh media
from time to time.

DVDs are less scratch resistant since there is more info densely
packed. I'd suggest putting them on cds then shrink wrapping them.

Offhand, I recall friends suggesting that CDs with gold-colored media
have the most longevity. When it comes to brand, I suggest that you
study the durability of the product labels of the brands you've
bought, if their product label side peels off easily. I've seen some
CDs the product labels of which peel off after some use, or because of
the heat in this country, and thus cannot be read anymore by the CD
reader, since the reflective surface underneath those labels that
reflect back into the CD media has been lost.

For long-term archiving it is best to use the highest quality discs
possible. These discs will have undergone rigorous testing and
generally have a guarantee of how long they will playback for. I would
suggest the following brands:

DVD Media: Verbatim, TDK, Sony
CD Media: Taiyo Yuden, Verbatim, TDK

Have a look at 
http://www.buysafe-online.com/Products.asp?objecttradersearch=1&GroupID=1124
  for cheap prices.

I also recently wrote this article about recommended media:

http://www.andybrain.com/archive/mb/recommended_media.htm

Thanks to everyone who has taken the time to respond to this question.

My own researches over the weeks has led me to believe that "Taiyo
Yuden" is the favourite brand of the majority of ITK users.

While TDK & Verbatim are frequently mentioned in glowing terms, TY
manufactured discs appear to have an exceptionally high level of user
satisfaction and loyalty and are consistently touted as an excellent
choice for archiving as well as for day-to-day reliability.

I think marky1979-ga's recommendation of different brands for
different jobs is quite astute but the feedback from TY users seems to
justify faith in the TY brand for both tasks.

The other often heard words of wisdom are that archives should be
"refreshed" frequently, either on new discs or onto/into whatever new
media will be available in the future.