What, save to memory stick? Yes, that's what I've been doing, mainly when I wtarted working on the six unlockable songs in case I messed up and missed an encore (cheaterhead) but also saved yesterday because my brother in law came over and we played multiplayer. It was fun and he was talking about getting the game now.
I've had something similar happen to me before with COD multiplayer. When my daughter and I played online multiplayer on the same TV, the next time I signed in I had all her loadouts, gun settings and perks. Didn't notice if it had changed my stats as well.