So - we've got customers who had been using the Exchange 2007 Transport Agent's OnRoutedMessage event as a place to rewrite From addresses. This broke in Exchange 2007 SP1 due to some tweaks we made to the transport pipeline to solve a variety of other issues. We've got a fix for this now in Exchange 2007 SP1 Rollup 4, which just released today:
RU4 introduces a new event: OnCategorizedMessage. This event is at the right place in the pipeline to do your From address rewriting. Enjoy!