Looking for Linux drivers for SDIO cards? There seems not to be much available yet. But here are at least some pointers.
From SD Card Association(http://www.sdcard.org/sdio/index.html): "The SDIO (Input/Output) card is an interface that extends the functionality of devices with SD card slots. A variety of SDIO cards are being developed, such as a Camera, Bluetooth, , GPS, and 802.11b."
C-Guys(http://www.c-guys.net/) (in Japanese) offer a WLAN SDIO card together with a Linux driver for the SHARP Zaurus Linux PDAs. For more information see this thread(http://www.zaurususergroup.com/forums/index.php?showtopic=510&st=75).
This site is an open collaboration effort for open source projects relating to the MultiMediaCard and SecureDigital and SDIO protocols(http://mmc.drzeus.cx/) . Currently it is mostly about the Linux implementations, but other systems such as BSD are welcome to participate.
SD-MMC(https://opensource.motorola.com/sf/sfmain/do/viewProject/projects.sd-mmc): Open source implementation of the SD-MMC device drivers.
LinuxDevices: MontaVista, Atheros(http://www.linuxdevices.com/news/NS6177996610.html) contribute Open-Source SDIO Stack(http://source.mvista.com/~dsingleton/sdio/).