Update: Valve has finally confirmed, that the problems were caching problems as written here:
Steam is back up and running without any known issues," a Valve spokesperson told GameSpot. "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.