It looks like the person with the n900 tested the 64gb card as a usb device with the n900 in usb host mode. The n900 only has a microsdhc slot so there is no way to insert a full sized sd/sdhc/sdxc card in the slot. The nice thing about the n800 is it has TWO full sized slots, making 256gb possible using 128gb cards. Today that would be pretty expensive but that situation will surely improve.

Exfat is apparently patented but Linux native file system are better anyway.

Apparently the current generation sdxc cards (SD3.0) are electrically compatible with sdhc but the next generation will be incompatible (higher clock speed among other things).