That sounds like bad news for Canonical. Phoronix's Michael Larabel summarizes the impact as follows:
Canonical will now need to carry the XMir support out-of-tree from the xf86-video-intel driver. Canonical is also carrying patched versions of Mesa, xf86-video-ati, and xf86-video-nouveau for being able to support Mir/XMir in Ubuntu 13.10. The binary AMD and NVIDIA graphics drivers also remain incompatible with Mir.
As I understand it from the forum discussion on the linked article, Canonical developers will need to make changes to each upstream release of the open source Intel, AMD and Nvidia drivers, and of Mesa, to make those work with Mir/XMir. While Debian based distros are of course no strangers to meddling with upstream releases before packaging, this sounds more involving.