I want to take a page out of Bill Gates' dream home, and flush-mount a
monitor (say 19"), surrounded by a picture frame, on one of our walls.
Question: Is my best bet to buy a 19" flat-screen LCD monitor, remove
its stand, and simply inset it in the wall whole-cloth; or, is there
an even cheaper source for the internal components of the monitor,
without the plastic case, that I might use?
One challenge is that our home has block walls (not timber framed), so
cutting our the niche will be something of a hassle. The thinner the
better.
I intend to have an ornamental cabinet below the "picture", and run
the power and video feed up a chanel that I'll plaster the wall. (What
sort of cables should I use to help ensure I don't "obsolete" the
system too soon.)
I was even thinking of integrating a motion sensing plug between the
wall socket and the monitor's power supply, so it will turn on only
when there's someone there to see it. (Sort of like a tree falling in
the woods?) Of course the computer itself will run basically all day.
Anyway, recommendations and brainstorms would be much appreciated!

There are plenty of ways to do this.  Yes you can get the OEM LCD
panel by itself, without a case, but you will still need to build
something around it to make it safe to mount in a wall (or even on a
wall).  Remember electronics mounted in/on a wall and the associated
in-wall cabling are a fire-hazard.  So be careful.

A good resource for this is a webforum called AVS-forum.

You can find it here:

http://www.avsforum.com/avs-vb/

Specifically the Home Theater Builder Forum

http://www.avsforum.com/avs-vb/forumdisplay.php?s=e600f756d19ba35d1f5e7d32197c2815&f=19

and the LCD flat-panel forum

http://www.avsforum.com/avs-vb/forumdisplay.php?s=e600f756d19ba35d1f5e7d32197c2815&f=40

will be of use.

Remember to register and ask your question, not do a search and hope it comes up.

-IR