A huge block-chain re-org on the testnet confused the heck out of the testnet faucet (leaving it with lots of 0-confirmation, used-to-be-valid sends that made it think it had a negative balance).
Clearing its wallet of the 0-confirmation transaction fixed it, but left it with a very small balance. It is up and running (at https://testnet.freebitcoins.appspot.com/ ), but if you generated a bunch of testnet coins, please send some to the testnet faucet at miGuMc6qtVEKS6Pf1jKddaa81DeHjMzkpB .
Also, for anybody who doesn't know about it already: I've been using testnet-in-a-box (thanks Mike!) very successfully for testing. It gives you a nice, well-defined, controlled environment for reproducing bugs. Get it from:
http://sourceforge.net/projects/bitcoin/files/Bitcoin/testnet-in-a-box/