BBPress is excellently
SEO'd out of the box. Clean URLs, Tags, Latest Posts on Homepage, Pagination, Uses CSS over Tables for Layout, Solid Page Title Structures, OS Development so Plugins are freely available, and more...(I presume).
Demo -
bbPress support forums
I'm going to get into BBPress for a site fairly soon. I haven't used it in over a year but the guys are nearing their 1.0 release fairly soon so I expect things to kick off for them.