Software to take Picture via iSight after some interval
I think Photo Booth, included with Mac OS X, has a time-lapse mode, but I can't check because I'm on a Mac without a camera currently.
If it doesn't, you can try an application called Gawker, which bills itself as having a time-lapse mode.
ImageSnap recently added the ability to take a picture at a specified interval.
You could then stitch the images together into a movie using 'mencoder' or a number of other utilities.