At the last minute ahead of next week's Ubuntu 13.10 debut, Mir is now deployed in Ubuntu Touch for the Google Nexus 4 "Mako" and Galaxy Nexus "Maguro" phones. Other phones/tablets are apparently still relying upon Android's SurfaceFlinger.
The latest Mir landing update was shared on the Ubuntu Phone mailing list. In looking at the latest Ubuntu Touch Mir image testing, there's still many failures happening. The very latest build at the time of testing has 51 of 150 tests failing and 34 crashes occurring during their engineering tests.