Not much yet, but we've succesfully identified the ASIC from HTC as the so-called asic3 as also used in the iPAQ h3900, h1910 and h41xx.
The ASIC is for GPIO and SDIO and is located at physical 0x0D800000.
At 0x0D801000 an ASCII part number can be found: HTC-SDIO P/N:30H80028-0'
Thanks to Philip Blundell
Well it seems this ASIC is not used for the SD interface, but one at 0x0C800000 (maybe ASIC5?)
also info on SD card procols etc can be found at: http://www.envoydata.com/secureDigital.htm