I read a topic on these forums a while back about people wanting to use the same DB for phpBB as they did for WP. Unfortunately, I was unable to do that... I don't use WP for comments anyways, I use phpBB.
But I did manage to craft a way to syncronize WP blogs with phpBB2 topics...
Here is how it works...
I post something in WP, and it creates a topic on the Blog Response category of my BB identical to my blog. On WP, the comments link points to the newly created topic on my BB. Essentially, I have made phpBB handle my comments rather than WP.
Many people just link to their bulliten boards if they decide to go this route, but I decided it'd be neat to create a post on that BB, so that people wishing to respond to a blog wouldn't have to start a topic, they could just respond to the existing one. I also setup phpBB in such a way that no user can create a topic in my Blog Response category, they can only respond to them.
I managed to do all this without any modification to phpBB2, however... because WP chose to use functions with the same name as phpBB2, I had to modify WP a bit more than I had hoped.
If anyone is interested in the how-to, please let me know. I would link to my site to show you, but I am still in the process of getting it online and running smoothly.