A couple months back, I was able to work with Quest Software on a new whitepaper for Exchange 2007 migrations. As you probably already know, Reader, Quest makes some of the slickest migration software on the market. They also make Quest Archive Manager, which offers (of course!) email archiving capabilities. Quest's notion, and the one I explored in this whitepaper, is that by deploying an archival solution such as Quest Archive Manager, you can actually reduce the risks you'll face during messaging migration. The paper is specifically about migrating to Exchange 2007; while I didn't focus on the details of Exchange migration, I do cover some of the possible risks you face during a migration to Exchange 2007.
If you're interested in reading the whitepaper, you can get it for free from Quest; you simply need to register your email address with them.