2nd-round invite to the start of "MilePoint.com" on one of XF's 1.0 betas. Randy Peterson invited a large core group to get things rolling, and I was swept up in the next round with an invite from my friend Dave H. aka Cholula. I realized something big & good was afoot when I noticed one of the names from vB (Mike, I think) was keeping close tabs on MilePoint, and that another (Kier) was also involved. That was about a year & a half after I'd sworn off any additional purchases of vBulletin software, basically just putting off any further forum efforts (other than one flop with phpBB) because I didn't lke my options.
Fast forward another 4 months and I was in a situation where I HAD to set up a forum (I was the only one with the technical expertise among a group of ~12 looking for a new home), so I gave XenForo a try. I've never looked back.