You need an updated SD driver to use the bigger cards. The 2.16 had it, but as you know that ROM was retracted. There is a thread over at pdaphonehome.com that has a driver though.
The link below points to the thread I was talking about. Some said the 2.16 drivers worked, so I put together a cab myself. Haven't tested it, but you are welcome to as I have a 2gb non-sdhc card. If you want to try this, I would highly suggest you do a backup first (ie Sprite or SPB Backup) to be on the safe side.
http://pdaphonehome.com/forums/ppc-6...d-working.html