Try a Canon CF card. Does that work ? If not, then return the camera. It is a warranty issue. And ask Canon for help. If a Canon CF card works, then use Canon's. Afterall, it is recommanded, and other brands "may not work properly". But do try other brands and sizes.
I just bought a brand new XTi camera, new lens, battery, everything, but I've tried three different CF cards and they all bring up the message "There is a problem with the CF card. Replace the CF card". Well I have replaced it. Three times. And it's not helping anything. When I try to format a card it brings up a Err 99 message. Someone please help, I can't afford to buy all these cards to see which one will work.