I had the same problem with my new SanDisk Ultra II 1GB card!! I registered it through SanDisk and got a replacement under warranty. I have not used the new one yet, but my soon-to-be brothernlaw is a computer programmer, he tried fixing it with no luck. We researched the web for other people who have had the same problem with this card and there were PAGES and PAGES of people complaining about it.
My card messed up right in my camera. I went to delete some photos and bam, it did that to me, right at Disney. Luckily I had my other cards in my bag with me, but I lost some good stuff..about 80 pictures or so!
On another note: I also have regular SanDisk 1GB cards that work like a charm - I have been using for a long time now. It is just the Ultra II that I had the problem with. <Crossing fingers that the new card doesn't mess up on me>