Yeah, if my 3GS was busted right now I'd really be in trouble. Can't get an iPhone 4 for nothing, can't even get an HTC Desire...all I could do is get another iPhone 3GS and that would reeeeally suck right now.
If worst comes to the worst, you can always pick up a PAYG phone for under 5000 Yen from Don Quixote and put your SIM in that. That's my very cheap emergency plan (assuming I can't get an old one noo ne wants from somewhere). Somehow it'd feel really degrading/backward putting your SIM into a phone that first came out 5 years ago, though.