DIY, definitely. You can pick up a big (4' x 6' I believe) door mat at Lowes or Home Depot for about $12. Lay it down in the back and just cut it down to size. Actually, here's a DIY in the Modification Index with photos.
http://forums.genvibe.com/zerothread?id=15734He didn't do the back of the seats, but you could easily. The carpet sits in there pretty well because of the contours of the bay, and I also used my cargo tie-downs to anchor it in various spots. Then I cut a little slot for the baby seat tie-down, and was good to go. No more groceries sliding all over the back. First and best mod I did on the Vibe