The ARM platform is exploding like a mad wet cat out of the bath. Here are four good distros cram-full of ARM fun. Linux has had ARM support since forever, but it's been bumpy. There are hundreds of vendors of ARM devices (see Tiny Pluggable Linux ARM Computers Are Red-Hot for a sampling), all shoving their own personal hacked code out the door as fast as possible. This made Linux support complicated and unwieldy, to the point that Linus Torvalds threatened to stop accepting ARM changes in the mainline Linux kernel.
So, in classic Linux fashion, vendors and developers banded together and coordinated and consolidated their efforts, formed the Linaro non-profit engineering organization, and performed a sizable cleanup of redundant and bad code. Jonathan Corbet, editor of Linux Weekly News, predicts that "ARM will take its place as one of the primary Linux architectures" in 2012. This seems a safe prediction as ARM-based mobile devices are going to continue to sell like ice water in hell.
Android is also expected to harmonize, over time, with the mainline Linux kernel. So this confluence of ARM events means that we who like to play with stuff will have a little easier time of it. Me, I dream of embedded and mobile devices being as friendly to experimentation as the x86 platform. Don't laugh, it could happen. So which Linux is best for ARM? Why, lots of them.