According to #bitcoin, looks like someone flooded MtGox with 0.02BTC nanotransactions, in addition to shooting off various other sites... which proves two things:
1) Bitcoin is too dependent on MtGox
2) While the bitcoin transaction system is difficult to be disrupted, the exchange/wallet services (MtGox/Instawallet etc) and the trading info sites (blockchain.info) are the "critical points of Bitcoin infrastructure".
So as long as we need these services to actually commit trading (and use Bitcoins!), we need external, vulnerable services.