i know of no usb-capture _card_ that is supported by linux (but last time i looked is >1 year ago)
yes, there are a few supported by video4linux ( was it winTV or something like that, the brand ? )
I used a USB winTV card under linux in 2001-2002. However, I may have just been lucky to have gotten one of the older ones without any propriatary software compression.
however, now that firewire cards are so cheap (and somewhat standardized) you should really think about that.
-august.