I too had almost the same exact situation as FlatFrogger a few posts up, and sentr1c a few posts up.
I have a 20gig 260, I have always played offline and NEVER moved my xbox anywhere. I have never disconnected my HD and have two profiles, one is a guest profile and my account. I played one local game with the guest account and I checked afterwards and still had all my weapons, gold, and animals. A day ago, my nephew was supposed to come over, so I purchased a 256mb memory card for the xbox. I made a COPY of the castle crashers game to the card, in hopes to use it as a backup if ever my game went corrupt as I have heard a rash of complaints on the web. I played as usual that night, no problems. I always shut down properly through the menus, etc. The next day I go to play and load up one of my characters ( they are all there and they have all their attributes and correct levels ), but to my dismay, when I go into the weapons frog and animal barn, ALMOST everything is gone. I have the one animal I was equipt with since the last time I played and the one weapon I was equipt with when I last played. I had a total of 7 basic weapons left in the frog, and no other animals, under 100 gold when I left with 3000. I was last playing with the kings scepter someone dropped for me ( could that be the glitch? ) other than that, everything else I have obtained in the game was from single player off-line local play. I am as you have guessed now pretty pissed off as well. I am wondering why more testing hadn't been done before release. Besides always having problems with connecting to people to play online (works 1 out of every 50 times), the data loss is a REAL PAIN IN THE balloon. I love the game, but I had almost every weapon except for the rubber sword and spoon, and had every animal and was basically looking forward to unlocking all the other characters. Now I hesitate to even continue playing this game. I truly want to support indy-type games like yours and 2D-Boy with world of goo, etc, but this leaves me disenchanted. I wish something could be done to remedy the situation. I have of course attempted all "listed" fixes in the official xbox 360 forums, but nothing worked. I am writing now in hopes my information can help pinpoint the problem so future users don't feel just that... used.