I still have some arm9 netbook floating around with Gentoo on it... but it doesn't look like anything is really supported. As a matter of fact, I cannot find any releases for arm since 4.0
Arm is quite well supported; you just aren't recognizing it.
There are over 6 "architectures" based on arm:
acorn26, acorn32, cats, evbarm, hpcarm, shark, zaurus, and a couple older ones (netwinder & iyonix are out of date).
Now get to work and find out which it is :P
(I have one of the cursed i810 chips in an old tower running NetBSD.)