Nokia N900 is actually the most open smartphone there is.
Some andriod-based phones are open, on others, there's no chance of
installing another OS.
I've heard of a couple of linux distros running on N900, and you can
actually dual-boot on it, which would be nice __while__ you're trying to
get openbsd running (assuming it doesn't work out of the box, which I
don't expect it to).
--
Hugo Osvaldo Barrera