My CC details were "leaked" by Steam (Score 1)

Coincidentally, I'm currently fighting a running battle with Steam support to reclaim a hacked Steam account. After about five messages back and forth, it has finally emerged that the person actually stole my account by "reclaiming" it from Steam, after providing my steam account number, and my credit card details.

I don't have any spyware on this machine - I checked with SpyBot and Ad-Aware. I surf using Opera, I read mail using Eudora, and internet security is part of my job. I am at a loss as to how anyone could have got both my Steam account number and my credit card details by hacking a third party, however, unless that third party was Steam. (Yes, I could be an idiot, riddled with spyware that I have no idea is there.)