SDIO is an interface designed as an extension for the existing SD card standard, to allow connecting different peripherals to the host with the standard SD controller. Nowdays it's extensively used to connect WiFi/Bluetooth chips on ARM boards, like Wandboard or Banana Pi. GlobalScale DreamPlug has an internal SDIO slot with the Marvell WLAN/Bluetooth module.
FreeBSD lacks SDIO support and is not able to talk to the peripheral devices behind SDIO. Work is going on to create a driver for the FreeBSD. Earlier version used DreamPlug hardware, which has Marvell SD8787 chip (WiFi, BT and FM).
Currently the MMC stack is being moved onto CAM framework, this will allow to handle interrupts generated by SDIO cards and hopefully fix several other issues with the old stack. The version that works supports MMC/SD cards, with SDIO parts in active development.
The old source code is kept here: https://github.com/kibab/freebsd/tree/kibab-dplug
Version of MMC stack based on CAM framework: https://github.com/kibab/freebsd/tree/mmccam
Wikipedia article about SD and SDIO: http://en.wikipedia.org/wiki/Secure_Digital#SDIO
SDIO specification: https://www.sdcard.org/developers/overview/sdio/sdio_spec/