dynamo52 sends this quote from Ars about a breach involving a Bitcoin exchange:
"More than $87,000 worth of the virtual currency known as Bitcoin was stolen after online bandits penetrated servers belonging to Bitcoinica, prompting its operators to temporarily shutter the trading platform to contain the damage. Friday's theft came after hackers accessed Bitcoinica's production servers and depleted its online wallet of 18,547 BTC, as individual Bitcoin units are called, company officials said in a blog post published on Friday. It said the heist affected only a small fraction of Bitcoinica's overall bitcoin deposits and that all withdrawal requests will be honored once the platform reopens."
Reader linhares points out a forum post discussing how the attacker(s) hinted at a 'mass leak' in the near future
. This attack comes shortly after a leak of a different sort — an FBI document
(PDF) about Bitcoin found it way onto the internet. It seems they're worried about the virtual currency's potential use in criminal activities