When you skipEvent() in preDelete(), the postDelete() is run outside of the internal transaction, whereas when you don't skip the event, it is run inside the transaction. It would be good if both were run inside the transaction so that you don't have to conditionally start a transaction in postDelete depending on if the event was skipped.

I think this is just a reordering of these lines:  http://trac.phpdoctrine.org/browser/branches/1.0/lib/Doctrine/Connection/UnitOfWork.php#L278