For the last half hour or so if you load up the steam client and click account details it would randomly take you to some other person's steam account details including their name, address, email, phone number, and last 4 digits of any saved credit cards, their amount in their steam wallet and purchase history.
Looks like maybe they've started fixing it now, but uh...this was pretty bad so watch out for stolen personal information for the next month I guess. :\
*edit* yeah, Valve's completely shut down Steam at this point, so at least no further compromising hopefully.