Hey everyone,
It looks like the site experienced a bunch of different people (or maybe the same one) creating new wallets but not depositing anything depleting the address pool. I'll have to fix that, but everything else seems to be working great!
The guy who runs the excellent '
flipside' bitcoin game had the same issue, and found one of my StackExchange answers helpful in understanding the problem. See here:
https://bitcointalk.org/index.php?topic=162712.msg2046657#msg2046657