Yes, both of these work:
cat /dev/video? >file.mpg
cat /dev/video? |tee file.mpg|mplayer -
"tee" gives preview, but it is jumpy even though I have a current technology mobo with 6-core 3.3g AMD 11T processor and 12g ram.
After I learned to use v4l2-ctl to set the correct inputs everything fell into place. I have an internal capture card so sometimes I get a different number for /dev/video?. But it does capture component video very well.
Thank you all for your help. After I get my DVB-S2 receiver card I'm sure I'll be back asking for MythTV help, unless I just do it all with Mplayer or VLC or something like that, but I'd like to be able to set timers, etc.