Dave,
I recently converted from FudForum to bbPress by way of SMF and PHPbb.
Convert the board from SMF to PHPbb and then to bbPress. Worked like a charm.
I had the same issue last night when upgrading to RC2 and WP 2.8. Try removing all of the integration cookie links from bb-config and wp-config and then reconfiguring them. I’m sure there is a quicker way to resolve it, but that worked for me.
