as I understand it...
basically the problem is that SDIO is a proprietary extension to the SD standard and is locked down by copyright and non-disclosure agreements. Although people are said to have sdio working on linux, they are afraid of being sued if they release it.
the only sdio driver that's available is a specific one for a wifi card, it's a kernel 2.4.18 module (meaning sl-c860 & 760), done by c-guys, and it's reported to not be terribly robust.
embwise have an sdio driver stack and claim it's fully supported on linux, but no price is available. I have fired off an email to embwise to see if maybe we OESFers can negotiate a group price but I just haven't had time to do anything.
hope this helps, sorry it's not good news