I have received one of the recently announced Lexar 64GB SDXC cards (there's a 128GB version also available).

The card shows up as exFAT formatted on my laptop and it is ready to use. Not so much on my old Nikon D50 DSLR.

As a side note, using this card for Windows ReadyBoost works, but I think I found its limit: even though I select "Dedicate this device to ReadyBoost", only about 32GB are actually used, leaving the rest of the SD card free.

I will post some speed tests later. Will also try and find a DSLR that supports SDXC cards.