Pingbacks not using permalink structure (with solution)

Sometime in the last two weeks, pingbacks from my blog stopped using the permalink structure and started showing up as path-to-wordpress/?p=x

It was sometime after the upgrade to 1.5.1, so I suspect it was the move to 1.5.1.1.

I can’t find anything with the new search, but in case anyone else has the same problem, I solved it by going into Options/Permalinks and just clicking on Update Permalink Structure without making any changes. I assume one of the changes in 1.5.1.1 required a new rewrite rule.