exFAT, aka FAT64, is a significant revision of the FAT32 design, for current and future needs (specifically, mobile storage like USB flash keys and the like). Lots of improvements beyond raw storage size, e.g. timestamps with 10ms granularity. The Wikipedia page also has a link to Microsoft's WinHEC presentation laying out the improvements and rationales in more detail.
I stumbled over it this morning and I have to say, I'm reasonably impressed.
Here's a benchmark from Jan'09 comparing exFAT against FAT32 and NTFS. Not bad considering exFAT's youth. I'm sure drivers (and hardware) will get more optimized over time.