They need reformatting to Fat32 in the device itself, but after that it works fine. Even works in my Galaxy S on Froyo, and my stock HTC Sensation - I suspect it will work in most SDHC compatible devices.
I've already nearly filled the card, oh well... look forward to 128GB cards next year? Better start saving now...
Lots more detail with benchmarks and so forth at AndroidNZ: Sandisk 64GB Micro SDXC cards confirmed WORKING in Galaxy S II, HTC Sensation, Galaxy S
Sandisk 64GB Micro SDXC Vs Samsung Galaxy S II - YouTube