Ubuntu has actually been ported to several phones. HTC Tilt and
Tilt2, HTC HD2, and a few others that I'm aware of. Generally as a
dual boot solution with the native phone OS and the port is by
independent developers (battery drivers are still buggy with bad
charge reporting in most cases).

Also, the motorola atrix runs ubuntu on it's lapdock and webtop by
default... though it is locked down to firefox and an image viewer,
when rooted you can install synaptic and get the full experience
(assuming you have time to wait around on the tegra2 processor to do
think and swap out memory as needed).

see also "how to install ubuntu on android":
androlinux.com/.../how-to-install-ubuntu-on-androi\
d/

google and search over at xda-developer forums and you will find a lot
of information on the topic if you have further interest.