People have asked here about compatibility for SDXC many times before. Theoretically it will work, but you'll need to reformat it with a supported filesystem, the Linux kernel driver for the card reader has supported the SDXC spec for ages so it shouldn't be a problem. There are also open source implemenations of ExFAT which might be able to be ported to the N900 should anyone care enough to.

I'm sure as soon as someone with a N900 gets a SDXC card they'll post back results here.