They work... but i've no idea if they work correctly.
We could not get anything to recognise the 8GB card (card readers etc) apart from the GP2X... and then all that was possible was to copy stuff over from NAND to the card which did work.
It's possible the GP2X will stop at some point using this method as it cannot get the card size correctly, maybe 2gb, 4gb etc. it also might crash or not read files after a certain point. Not enough has been done to test it.
Eventually the GP2X will correctly support them, it is only a software issue.
It's on the list to be investigated further