Trouble with the ffmpeg -ss flag when capturing one frame from a Macbook iSight webcam

For about five years I've used ffmpeg in a shell script to grab one frame from my linux'd-up Macbook's iSight:

ffmpeg -f video4linux2 -s 640x480 -r 15 -i /dev/video0 -an -vframes 1 -vcodec mjpeg -y -sameq -ss 1.5 snapshot.jpg

I just upgraded my Ubuntu distro from Lucid to Natty (ffmpeg ver. 0.6.2-4:0.6.2-1ubuntu1). Now that syntax turns the iSight on but hangs indefinitely.

[output snipped, ending with:]
frame=    0 fps=  0 q=0.0 Lsize=      -0kB time=10000000000.00 bitrate=  -0.0kbits/s    
video开发者_运维技巧:0kB audio:0kB global headers:0kB muxing overhead -inf%

Without the '-ss' flag it seems to successfully grab the first frame and exit immediately - the only difference in output being:

frame=    1 fps=  0 q=0.0 Lsize=      -0kB time=0.07 bitrate=  -2.6kbits/s    
video:16kB audio:0kB global headers:0kB muxing overhead -100.132730%

However, the '-ss 1.5' was necessary to delay the frame capture by 1.5 seconds to allow the cam sufficient time to adjust the exposure.

The -itsoffset flag seemed promising, but doesn't seem to change ffmpeg's behavior (ie doesn't hang, but no delay).

Any ideas?