I'm looking to integrate Invision Power Board's user database with WP, and replace WP's authentication process. Now, I understand WP login is all pluggable, that's great.
The makers of IPB have actually made this easier on their end as well by creating some software called Converge, which can centralize the login authentication for many different applications using XML-RPC method calls and method responses.
So, I imagine it's just a matter of replacing the WP login functions with the XML-RPC calls required by Converge. Unfortunately I don't have the necessary PHP knowledge to make this happen.
If there's any adventurous WP guru out there that wants to take a stab at this, here's the Converge documentation: http://forums.invisionpower.com/index.php?act=attach&type=post&id=11100, and I can provide the "coverge_local" files for further examination. Let me know!