I just integrated a garage door monitor with an imp and I have to say I’m very satisfied with the results. I used a chamberlain garage door monitor ($19 at amazon) and a Imp with a April board to detect the status (open/closed)
The monitor has 15V so hooking it up to the april converter powered it perfectly. The only trick thing (which wasn’t that trick) was the ‘open’ signal I pick up is off the LED and the most convenient pin is either 5V (garage door open) or 2.2V (garage door closed). I did a small resister divider to get it to < 3.3V and used an analog pin to decide which state it was int.
I’m still playing with software, it’s pretty simple right now and mostly logging to cosmo but the one tricky thing was my first version was waking up the up way too often to check status and I think all of that activity was interferring with the RF signal used to detect garage door state (the first 12 hours I went live I kept on knocking the monitor off line in the sense that it thought it couldn’t detect the remote portion)
After a quick sw change to have the imp wake up and check less often and spend less time frantically uploading status to the server fixed it – It’s worked prefectly since I made that change (almost 24 hours now)
A few more details & pictures are in some google+ posts