Maybe more than you want, but it's freeware so worth a try:
http://www.thugsatbay.com/zweistein/
This one is also pretty popular:
http://www.mpegx.com/view.php?detail=432
I haven't used either, since I actually had to buy a pro-editor for my uses. But they loook like they'll do what you want. I'd try the second one first.
Unfortunately, you want to go mpeg to mpeg. If you wanted to go mpeg to AVI, I'd highly recommend Virtual Dub for its straightforward simplicity and used it a lot recently.
Here's another one that might work, but I don't know if it saves Mpegs:
http://www.mpegx.com/view.php?detail=1633
You didn't specify if you want to select a 30 second clip, or a bunch of shorter clips you're trying to join together for 30 seconds total.