bitcoin/bitcoin — pull request #686 by gavinandresen (2011-12-05 22:03:36+00:00)
Title: Testnet difficulty calculation changes, to take effect Jan 1 2012

Allow mining of min-difficulty blocks if 20 minutes have gone by without mining a regular-difficulty block.
Normal rules apply every 2016 blocks, though, so there may be a very-slow-to-confirm block at the difficulty-adjustment blocks (once per month, assuming testnet is in it's normal "difficulty too high for number of people mining it" state).

This will almost certainly cause a testnet blockchain split after Jan 1. I'll update the Testnet Faucet, I'll ask theymos if he can update the testnet block explorer bitcoind.