Unless a Micro SDHC card is actually produced we can't know for sure.
However, considering how expensive such cards are likely to be at launch, I would be hesitant to fork out the money and stick one into a device that doesn't advertise compatibility
less I risk damaging the card's contents with too low a write speed or having it just plain not work at all.
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
I'm sure as soon as someone with a N900 gets a SDXC card they'll post back results here.