Sandisk now seem to have released a 64GB Class 10 microSDXC (as of May 2012) , which is available for pre-order at £45 from Play.com, due on May 15th. This seems almost too cheap to be true, yet Amazon are offering the same for £50 (subject to stock).
Based on previous good reports with the (slightly more expensive) 64GB Class 6 from Sandisk I've decided to take the plunge on this one, and I will of course report on N900 compatibility as soon as it arrives...
I' am not a power user, and I'm not a developer but I do know how to reflash N900 if something goes wrong.
So I own 3 Nokia's. One of them is for testing and reflashing, one I use for my job and one of them is for parts but it is flashed with latest image and nothing else is installed on it. I bought 64GB SDXC-UHC San Disk card and when I put it the phone that has all kinds of software installed, nothing happens, not even errors that some of the users report, nothing at all.
So if You have that kind of a problem, and U do own another nokia that has basic image, it will be recognized with that error "unsupported operating system" and U just format it as described and put it in the other Nokia and it will work fine. Why, I do not have a clue, but I suspect it has something to do with kernel I'm using (CSSU-test). Also you could try formatting it with some kind of third-party software.