I've got a Lexar 12x 256MB compact flash memory card and I've never had any problems. First started using it with my old sl-5600... then with the sl-6000L and now with my C3100.
Works when I need to do rom flashes, kernel updates, NAND backup, and NAND restore. Cost me way too much when I first got it but I can't complain as it's never failed me, not once. Sorry to hear the kingston card doesn't seem to work. Makes me a bit leery about getting a higher-capacity card for fear of it not working... *sigh*