Not 100% certain, however from what I can tell in the access logs, they used either an XSS or and CRSF exploit to get in. There were at least 2 plugins on the site that were vulnerable as well as an older template. We were also one point version behind on the forum software.
So with the vulnerable areas they were able to use an SQL injection in order to change settings and get elevated privileges, once they did that, the world was their oyster so to speak.
To recover I had to locally restore a backup that I took before the attack, I then had to transfer that data (posts and user accounts) over to a new install, then upgrade the software. Once that was done I had to manually reset permissions, strip out any injected "<script>" tags in signatures, posts and other areas. I then had to install a few security addons and then take another backup of the site to make sure that I could recover at a better point in time if it is hacked again.
I am continuing to work on tightening security more today