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Just to make this clear for anyone reading this thread, following Nachteuel's instructions do work and solved the problem for me. You can auto-mount exFAT formatted drives. Peiote made a typing error which seems to have caused his whole system not to startup (so be careful what you type!).

In a nutshell:
You will probably be told they're already installed an up to date, this is just to make sure they're there.
Or whatever editor you prefer. Add just one word to the bottom, if it is not already there, on a line on its own:
fuse
Then save and exit the file. It should look like this:
This tells your system to load the fuse module into the kernel at startup. Reboot your system. Your exFAT drive should now auto-mount. Peiote wrote "modprobe fuse" not just "fuse".

Why might you want to use exFAT?
If you require the Holy Grail of a USB drive that can: go between Windows, Mac and Linux; is read and write on all those systems; can handle files bigger than 4Gig; and requires no 3rd party software to be installed, then exFAT is for you! If you don't have such requirements, then NTFS (read only on a Mac), or HFS+ (unreadable on Windows), or ext3 or ext4 (unreadable on Windows or Mac), or just plain old FAT32 (read and writeable on Macs and Windows but limited to 4Gig file size which starts to become a problem with HD video files) may be easier for you.

Thanks @alcina for clearing this one up.

Another consideration for using exFAT is that it was designed with SD cards in mind.
exFAT does less writes to the SD card as the other filesystems do (no journaling etc) so the SD card can last longer.

For the sake of completeness:

With kernel build tonight exfat driver is now available as kernel module. That means, fuse module is not needed anymore and mounting of exfat formatted hdd's works now out of the box for all future image builds.

Unfortunately, if package exfat-fuse is installed (that's the case for all existing installations), this package has to be removed manually to make kernel module work.