Re: Upgrade advice needed.
In general I try to keep up with WordPress releases as well. The upgrade from 2.5.1 to 2.6 included these bug fixes:
In 2.5.1 the AUTH keys thing was really sorted out, so they changed that in 2.6 and that’s the main reason 2.6 is not compatible with the 0.9.* branch of bbPress.
If you don’t require integrated logins between the forum and the blog, then you don’t have to upgrade your bbPress to use the latest WordPress. If you do need integrated logins, then you probably need to wait for the 1.0 branch of bbPress to upgrade WordPress to the latest.