I would do a search for R/C video cameras. There are a whole range of miniature, lightweight cameras designed for radio controlled (R/C) cars, airplanes, etc. Many of these are wireless, a feature that might give you another degree of freedom.

You might also check out the various home automation sites. They also feature small cameras that are designed to hide in small places. Here's a link to my favorite site:

Smarthome

I have contemplated getting a small helium balloon from Costco (they sell the helium tanks) and putting one of these aloft in order to get very close "aerial" footage of exteriors. I've never had the time, however, to set it up.