About two years ago I was the lead on an Active Directory migration/consolidation project where we collapsed several domains and restructured two others.  It was a monster of a project and I don't know that we would have had the success that we had without the use of the Active Directory Migration Tool (ADMT) and the Migration Guide documentation.  It looks like there is a new version of the tool (version 3.1) and Tim Springston has all of the details at his AD blog.  If you have a current migration project or have a project on the horizon check out the post here and get a head start!