I'm looking to eliminate freestanding speaker stands in my living
room, and mount my Phase Technology bookshelf speakers on the wall or
ceiling.
 It looks like most speaker brackets require drilling into the cabinet
of each speaker - does this interfere with sound quality?
 Is there an online store someone could recommend that carries a good
selection of speaker mounting hardware?  Especially a store that has
knowledgeable people to call on with questions?

A scientist would say that anything that you do to a speaker enclosure
will in some very small way change the speakers sound.  I do not
however believe that you should be afraid of that.  Sound quality is
very subjective.   It may be that in  few years someone will be
selling speakers with metal inclusions in the back plate because of
some percieved benefit.  I say just follow the instructions, drill the
holes, making sure to not leave any mistakes or extra holes open, and
just get it done.  You will not notice any important difference (or
probably any at all). Dont be afraid of it.  Nothing to it...but to do
it.

I would attach the mount to the wall first.  If it's a plaster board
(drywall) wall you need to use a "molly" or "butterfly" or some other
device intended to take pressure off of the threads.  Logically it
would help to hit a stud with two of the (usually) three screws but it
is not required.  There will be instructions with the mounting arm.
(If you need to, you can ask another question about finding studs...).

Much will depend upon the weight of your speakers.  It doesnt look
like that phasetech company sells really big ones.  The mounts are
always rated by the weight of the speaker so have a good idea of what
your weigh before you buy yours..

After you have a good attachment to the wall I would then attach the
speaker to the arm.  Most mounts that I have seen allow you to
seperate the two ends of the mount so that you can just join them
after you ahve done the carpentry bit.

I have seen good quality speaker mounts at Radio Shack and Fry's and
at all of the big retail outlets that sell televisions and stereos. 
Good guys, best buys, even Sears..

As far as good knowledgeable help available....what planet are you
calling from?  After reading this you now know more than nearly all of
the employees at the average retail outlet.

Excellent advice, I'll follow it.

>As far as good knowledgeable help available....what planet are you
>calling from?

Well, there are audio stores where they really do educate their employees.
And there are online stores where you can chat with or call employees
for expert advice.
I know they're out there...

But I'll stick with the advice you gave me... thanks.