If indeed the new 3500+ chips are a full 1024kb of L2, it may mean AMD is going to phase out 512k chips from their premium line. Now I couldn't make any decent business case for this other than to compete on a checklist against Intel's 2MB chips (joe-blow wont understand IPC, Intel understands).
However, its probably more likely that its a failed SD 4000+ or such. A full fledged 1MB chip would be an awesome little overclocker though. :)