SD and MMC cards of any size can be used with ease. Note that the mainboard has to support FAT file system. FEZ Spider Mainboard supports FAT file system and also supports any SD cards of any size, no 2GB limit with SDHC support. This kind of SD modules makes use of the maidboard's 4-bit SD bus to access the memory card unlike the SPI based (1-bit bus) access on other device which is relatively slower.
Also GHI adds a way to mount the SD card to the USB device connector to emulate a USB mass storage device.