Well my Kingston normal 1gig CF gave me the same problem few times too. What i've learned was to format the card in the camera before use, seems to work fine for me. Weird thing is that it only happened on my Kingston and not sandisk cards.

Hmm.. when i lend cards to my cannon frens they always format it first, i do the same thing when i get back my card before i start shooting, just for safety.. i would be intrested to know if there are any real issues with sharing cards between different brands too.