Check out two really excellent blog articles direct from Microsoft BizTalk devs.
Kevin Smith has published a part 1 of an article on BizTalk ACKS and NACKS. Having fairly recently discovered the delights of NACK SOAP fault messages, this really filled in some of the gaps in my knowledge.
Lee Graber has also published an article on Zombies. Again, a really relevant subject. I remember the 'Completed with Discarded Messages' status of suspended services really confused me when I first saw it. It's great to get some insight into the BizTalk team's thinking about future features.