An exFAT (Extended File Allocation Table) file-system implementation for the mainline Linux kernel doesn't really stand a chance of seeing a mainline status due to Microsoft burdening the software with patents. Fortunately, thanks to FUSE for being able to host a file-system from user-space, a sufficient implementation has arose.
Linus Torvalds and others in the past have characterized FUSE file-systems as being for toys and misguided people, but FUSE has been used before for bringing Sun/Oracle's ZFS to Linux, various other creative file-system implementations, and now exFAT. ExFAT support for Linux has been talked about going back to early 2009 but the support has been crap on Linux.
The FUSE-based exFAT project seeks to be a full-featured implementation for GNU/Linux and other Unix-like systems, including Mac OS X. With fuse-exfat 1.0.0, after three years in development, there is support for formatting exFAT partitions using the exfat-utils package while the FUSE driver does provide both read and write support for the Microsoft FS. Some of the recent changes found with the 1.0.0 release include improved write performance through enabling big_writes, improved OS X support, and various crash fixes.
The only other Linux support for exFAT up to this point has been through commercial offerings from Paragon and Tuxera for Linux and Android, albeit not free and open-source. This fuse-exfat implementation has been released under the GNU GPLv3. More details on this user-space file-system implementation can be found from its Google Code page and the 1.0 release announcement.