With my Logitech Quickcam Pro 9000, I use this:
Live video:
mplayer -fps 30 driver=v4l2:width=960:height=720:device=/dev/video0 tv://
Or to grab a single jpeg frame from the command line:
mplayer driver=v4l2:width=960:height=720:device=/dev/video0 tv:// -nosound -vo jpeg -fps 1 -frames 1