I bought an SDIO wi-fi card for my Palm nearly 3 years ago, and they were available for PPC (the old pre-winmob PDAs) before that. SDIO never took off though, probably because it turned out too difficult to support compared to the physically larger CF. There were only ever a couple of types and very few makers of SDIO cards, unlike CF I/O cards where there were probably half a dozen types or more, and lots of makers.