For information's sake (taken from the Contribution thread):
Nayleen and I spent two weekends and the week in-between making the transition to both phpBB and SMF. Neither phpBB nor SMF offers direct conversion from IPB, and our test required a vBulletin intermediate conversion (supplied by Qberty). A lot of data was lost or corrupted in the move. Not to mention that after the conversions were done, the overwhelming consensus was that the community didn't like either option. After 120 man-hours of work, Nayleen and I realized it would take another 75-100 to restore lost functionality to either platform, with an unknown amount of bug-fixing and troubleshooting. At that point, time is money, and we'd both sacrificed too much "in real life" to justify taking more time with this project. Not to mention loss due to the forums being offline.
That's a lot of the story, but truthfully, many more factors went into the decision to return to IPB. I didn't make the decision lightly, but I feel the overwhelming positive response has justified it.