My first main project for my Pi is a garage security system. I've got an electric garage door opener so I'm interested to hear how this progresses.
My main idea is sensing when the doors are opened (either with a key or by force) and then taking one or more actions :
- emailing me
- capturing web cam images
- storing images on the SD card
- storing images on my NAS
- storing images on my website
- mobile optimised "status" page on my website for remote viewing
- light sensors to determine if the lights are on or off
- flooding the garage with gas (this is more a "nice to have" rather than a firm requirement)
- nuking the site from orbit (see above)
I can do all I need so far apart from the webcam capturing. Switches, LEDs, email and website is all do-able.
One word of warning about garage door openers which I learnt the hard way. Do not leave your car unlocked with the opener on the sun visor. Otherwise some opportunist scum bag can get into your garage and steal your bike. grrrrr
Thus my motivation to over engineer a device to capture images if the doors are opened

In terms of interfacing with the electric door I think there are two options :
1) dismantle the opener and interface a relay directly to the electronics inside
2) use a spare key-fob to interface to the GPIO and perhaps power from the Pi to eliminate the battery issue.
My Raspberry Pi blog and home of the BerryClip Add-on board : http://www.raspberrypi-spy.co.uk/