iTunes has an option to set start- and end-points for a song, which you can use to re-rip just a section of a track.
I'm pretty sure you could write an AppleScript to do this, so, if you're running Tiger, you could probably build an Automator script to
1) take all your selected files in Finder and open them in iTunes
2) run an AppleScript to change the end point to 30 secs
3) re-rip all those songs to mp3
4) move all those re-ripped songs to a specific folder
5) ...
6) run another AppleScript to revert the original songs' end points to maximum.