I've spent a little bit of time implementing the start of board files for the arm port. The initial push has been for the at91 subport only, and many improvements could be made to this. I've written up my initial thoughts on this on the FreeBSD wiki FreeBSD Arm Boards. It could use much improvement, I'm sure.
One idea that hasn't been reflected there yet, was shown to me by Sam Laffler who suggested using linker sets to allow boards to 'probe', 'init' and other standardized functions. This is an interesting idea and I plan on working on adding it to the above links when Sam has results to share.
I'd also like to expand the above wiki page to be a 'best practices' guide for all architectures where there's great diversity of boards/cpus/etc (eg, not the homogeneous env that x86 provides).
I'm also soliciting comments on the above boards in addition to the above. Send them to me, or post them here.