How to use dvgrab to video capture from a laptop/USB webcam?

I expect dvgrab to be smart enough to auto-detect my webcam, but

dvgrab - | mplayer -
Error: no camera exists

So I try the man page USB webcam option

$ dvgrab -V     

Error: v4l2reader.cc:66: In function "virtual bool v4l2Reader::Open()": "m_fd = open( m_device, O_RDWR | O_NONBLOCK, 0 )" evaluated to -1
v4l2reader.cc:66: errno: 2 (No such file or directory)terminate called without an active exception

So I try the other example in the man page for webcams

$ dvgrab -jpeg-over -jpeg-w=320 -jpeg-h=240 -d smpte=1 webcam.jpeg
Error: no camera exists

So I try with the input device, and this time the camera light DOES turn on

$ dvgrab -input /dev/video0 -
Waiting for DV...
End of pipe

"": frame dropped: timecode 00:00:00.00 date 1999.11.30 00:00:00
This error means that the ieee1394 driver received an incomplete frame.

So I try with 'USB webcam mode' and the input device

$ dvgrab -V -input /dev/video0 -
Waiting for DV...

And no other error, it just gets stuck... What am I missing?