PalmInfoCenter has published a howto on moving the Palm Treo 680′s slot driver to the Palm Treo 700p in order to enable SDHC support on it:
Commenters report success on Treo 650′s, too – what about trying it out and telling us how you fared?
EDIT: Don’t try it on PDA devices if you don’t have a backup. It was tested on two PDAs (TX and TC) where it caused a hardreset / reset loop !!