I wouldn't give up yet...
There are few things you can do. First, look on eBay for a bottom case for a 12" PowerBook, and you may just find one. They're rare, but they come up sometimes. Replacing it yourself would be difficult, but you could probably pay an Apple repair center to do it for you--I don't think they would charge too much. Find one of those funky little businesses, and they will probably do it for a fair price.
If you can't find one on eBay, it's probably worth the couple hundred dollars to get a new case from a third party, because then you could always sell it for more when you want to upgrade to the PBG5. But before you buy one, make sure to find someone who will replace it for you (unless you think you can do it yourself, which I've heard is really difficult on the 12").
Also, I once had a friend who dropped his TiBook, and he took a more devious route to getting it repaired. He somehow convinced Apple that the damage was caused by a power burst or something, I don't remember exactly what. He made up some crazy story and sounded distressed enough, so the rep told him to send it in and they replaced a bunch of parts, including the case that had been warped. I don't really suggest you do this, because the damage was your fault, but it's an interesting story nonetheless. Good luck!