Most smartphones like Sony Xperia, HTC HD2, Nokia N900 etc have enough processing capacity to run modern operating systems like Linux, Mac OS X. We have seen Nokia N900 hacked to run Mac OS X & Android in dual boot. Also Xperoid android ROM was recently released Sony Xperia X10 to let you to run Android on Sony Xperia.
Now here is yet another achievement from Linux hackers, fatsal from XDA forum has managed to run Ubuntu Hardy Heron 8.04 on Xperia X1. Though the functionality is very limited with Ubuntu 8.04 on Xperia X1 but it has opened whole new world for hackers and developers. If you want to run Ubuntu 8.04 on your Xperia X1 then head towards XDA discussion.
Ubuntu 8.04 on Xperia X1
P.S. You must have patience to download around GB of data before proceeding for actual installation.