I think OP should have returned the phone within his 30-day trial. Sounds like he's beyond it now and is full of regrets. Not much we can really do.
OP: One option is to go and get a go-phone as a replacement. Tell the sales rep that your phone was stolen (if you don't have insurance)...if you have insurance, tell him you broke the screen and don't want to pay to have it fixed...and you want to use the go phone as a replacement until you're eligible for an upgrade.
I saw a really nice go-phone for about $80 with a physical keyboard. Looked almost like a BB in design. BUT -- this is the kicker...if you got your Captivate through a 3rd party (not AT&T), then you'll need to keep your data plan on the phone until 6 months have passed from date of upgrade. Otherwise you could be dinged for $250+ fee.
If you got your upgrade through AT&T, you should be able to just drop the data plan or reduce it to whatever is needed for the go-phone.
The store rep should be able to help you with that...
Good luck.