I've got an Canon EOS digital rebel. says "no CF card" when there is a card in the slot. card works (tried it on another camera). thoughts?

The camera isn't seeing the card inserted.

Have you tried another compact flash card in the slot to see if your camera is definitely at fault?

If so get a maglite and examine the slot the card plugs into and check none of the pins are bent. If there is a bent pin you may be able to straighten it with a pair of tweezers, though you may need to dissasemble the camera to do this. If the pins are all ok then you may have a board fault and while this is fixable it requires patience, a steady hand, knowhow, and test equipment. It would be easier to search for a scrap camera of the same model on ebay with a working board (search for one with a broken screen or lens) and simply swap it out.

Of course before you do any of these things check if your manufacturers warranty is still valid.