No info on whether it is a DB hack or Steam bug etc, but if you go into your Account --> Account details, you'll be in someone else's account and can check stuff like purchase history.
I suggest to switch into offline mode. You can't remove your CC information since you can't access your own data, but you could notify your CC company to refuse any transaction. Also, if you use your Steam password on another site/account, change it immediatly.
If you enabled pre approval payment on PayPal, go to PayPal > pre approved payments > Valve > Cancel.
Edit: it seems like Valve finally took Steam store and account servers offline.
Edit2: Valve's statement to Kotaku:
Steam is back up and running without any known issues. As a result of a configuration change earlier today, a caching issue allowed some users to randomly see pages generated for other users for a period of less than an hour. This issue has since been resolved. We believe no unauthorized actions were allowed on accounts beyond the viewing of cached page information and no additional action is required by users.
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