Simple apartment security system 'just because I can...'

I was just thinking, what hardware and software time and $$ investment 
might I have to make to cobble together a simple apartment security 
system 'just because I can...'

That is, as all access to my apartment is through one room, setting up a 
motion sensor high in one corner, hooked to a nearby PC, the later 
sending an email / SMS whenever it senses movement in the room when I am 
at work... and a web cam that I can look at when I get the email - and 
then act appropriately... (phone call, etc...)

I am not particularly worried that my apartment is going to get busted 
into when I am at work, I would just like "a project" to work on...

I am learning and using .NET at work, and so thought this would be a 
nice mixture of 'geeky' and challenging...

Think it is viable? Are there wireless motion sensors there that I can 
'program' and/or write code based on responses from?

Just throwing an idea around, so was interested in anybody's thoughts 
and ramblings...

Now, stage 2 would be the Rhoomba entering the room and releasing a tear 
gas 'bomb'!!   :-)    (aside - did anyone see the Rhoomba they made 
controllable via Bluetooth, put a frog 'skin' on it, and then tried to 
control crossing a road in one piece from their apartment balcony - 
Frogger-style!?! hehehehe - expensive but fun!)

GREG...