Sorry for the delayed response, its been crazy on my end.
You've obviously done your homework and have a very solid plan.
I mentioned the security concern as so many ppl overlook it during the planning stages.
I like the powerline modem idea, as it allows the use of existing wire as a network infrastructure.
One exterior plug/outlet "could" allow physical access though, some thing to consider.
Schematics for a basic powerline modem are somewhere on my machine, but I've never got around
to ever building my own.
Your encryption scheme seems solid and well thought out.
I don't feel like you need any advice on network security...
PIR motion sensors indoors are a good choice if pets are not an issue.
They are cost effective, and cut down on false alarms that are normally
more prominent in video based motion detection.
However, I have found the "Motion" Linux utility to be among the best video based motion detection software,
if setup properly and running on a good machine.
Centralize as much as possible, don't forget about the breaker box!
You could strategically place a few micro-controllers here to handle most of
your switching needs. Adding in another box beside, above, or below would be
preferable to working in the existing enclosure.
Check local building/construction codes too, Where I'm at, we don't have any yet regarding residential automation.
A visit with the local fire marshal might be beneficial as well.
I've been running xbmc for my media-center a long, long time.
I'm curious now, what front end are you running?