Basically, here's the situation:

• In terms of hardware, SDXC slots are exactly the same as SDHC slots.
• Linux (including Maemo) has supported the extra addressing bits needed for SD cards larger than 32GB since SDHC support was originally added to it.
• The FAT32 filesystem, as used by SD and SDHC cards, can't grok storage larger than 32GB, so SDXC uses a new Microsoft filesystem: exFAT. Windows Vista SP1 and above support it.
• Yes, Microsoft does demand royalties from everyone else who uses exFAT. Thanks for asking!
• A company named Tuxera has signed contracts with, and paid money to, Microsoft for exFAT and has developed exFAT support for embedded Linux systems. The driver isn't free by any meaning of the word.
• Android uses Tuxera's solution and was the first mobile OS with SDXC support.
• MeeGo uses Tuxera's solution. Really.
• SDXC cards reformatted with other, Linux-supported, non-exFAT filesystems will work fine in Linux, Maemo included. Technically, the cards are no longer SDXC cards though, and they won't work in devices or computers that don't support those alternative filesystems. So, 64GB of videos on an Ext3-formatted SDXC card popped out of your N800 will likely completely fail to be recognized in the SDXC slot of your Windows notebook/television/Blu-ray player/toaster.

My understanding of SDXC is that unlike the SD/SDHC hit, the current version of the SDXC cards (Using SD3.0 format) should work in SDHC readers if they can talk that format (may require a flash update). There is some doubt as to the capability of them to read the (as yet unimplemented) SD4.0 format though, even with firmware updates.

One thing to note is that technically the capacity for SD (SD1.0) was 512M, but using some tricks (changing sector sizes mainly) you can boost that up to 4G (using the SD1.01 format). The same trick can expand SDHC up to 2T, so SDHC is not necessarily capped at 32G. SDXC is capped at 2T, and the SD3.0 definitions show the most expansion using the current tricks would be 16T. The SD4.0 format for SDXC may expand it up that high, but that's the current limit for that format.

FAT32 can use partitions larger than 32G just fine. Even windows machine can read/write such partitions. There an artifical 32GB limit to formatting FAT32 partitions in Windows XP and later systems. Linux and Windows third party tools have no problem formatting largerthan 32GB partitions to FAT32.

Windows puts an arbitrary limitation on *creating* FAT32 partition greater then 32GB, for a simple reason. All sorts of simple operations start taking ridiculous amount of time, like calculating the amount of free space. If you take a ~1TB sized partition and run the windows dir command, you will see that it stays stuck on the last line for a bit to calculate the free space.