I concur with fanoush, it's probably a 4-bit SD interface in there. The 1-bit interface is typically the I/F that's integrated into some of the XSCALE cpus. In my Tungsten T3 the CPU has this interface, but there's an extra SD interface chip too that is used instead, this one provides the 4-bit interface. IIRC the Dell Axim 30x (also an XSCALE device) uses the built-in 1-bit CPU interface, but don't take my word for it - I haven't looked at the Axim HW myself. I have looked at the T3, and it has a dedicated SD interface chip. If anyone of you are willing to look inside the Nokia you may find out things.. I'm not going to, hopefully my N800 arrives tomorrow (unless they sent it by truck for the last 2000km, which I suspect at this stage.. grr!) and I won't void the warranty :-)
I googled a bit and found this page: http://mmc.drzeus.cx/wiki/Controllers
The chip in the T3 is the W86L488 SD/SDIO Winbond chip.

You can buy MMC Plus cards many places now, and they promise to be twice as fast as SD, but I haven't seen many devices that support them (in Plus mode) yet. Some of the high-end cameras, possibly. My pocket camera handles it as just another SD, apparently.