do shell script "/your/path/isightcapture imagename"
Replace /your/path/isightcapture with the full path to the iSightCapture program, and replace imagename with the name you'd like to use for captured images. Save the script as an application, and call that program when you want to capture the iSight's output.
This forum thread over on the macosx.com forums contains more details, including info on how to set up a cron task to automate screen grabs, as well as a way to stop your little program's icon from showing up in the Dock.
This works as advertised, except I used a script I had made before that took the picture and uploaded it to a server (from a previous hint here). I just set up the app to run at login, and it now works just as it did before.
[robg adds: The home page for iSightCapture seems to have vanished, which is why I haven't linked to it...]

