I removed Xobni and no corrupt pst's since.
I determined that it was possible that I had an undetected corrupt ost file before the Xobni install. I'm giving it a whirl again.
So far, so good. No additional Outlook corruptions. The feature I like the most about Xobni is the quick access to the phone # for the sender and the list of files you've exchanged with them (and can click on to open).