As noted by Joystiq, trying to access your cloud saves is resulting in a 0x807b0198 error for many, although other parts of XBL like streaming video services and multiplayer gaming profiles are unaffected.
In June, a programming error at Internet-based storage firm Dropbox Inc. temporarily allowed any password to be used to access any user account on its website, though Dropbox says fewer than 100 accounts were affected.