Does the file "mycapture.mpg" already exist? If so you may be running into the "no clobber" shell feature. You could try turning noclobber off, or, have the name of your mpg file be unique each time the script is run...
Code:
cat /dev/video > mycapture-`date +%Y%m%d`.mpg &
The date command by itself simply prints out the date in the form YYYYmmdd. You can add a minute and second to that if your command runs more than once a day.
So
outputs...