It's a bad idea to try to break the "in-production" bitcoin network.
If anybody is starting serious work on either extending Bitcoin or developing compatible implementations or trying to break it by creating bad transactions, I think creating a "parallel universe" test network with its own block chain, data directory, etc makes sense.
Satoshi: would you be open to a --testnetwork (or something) flag to bitcoin that swapped to an alternate genesis block, data directory, listen port and IRC channel? Maybe with a really short average block generation time, too (like once per minute instead of once per 10 minutes) so everything happens ten times a fast to make testing quicker.