In preparation for our migration to SharePoint 2010, the first step is to run the MOSS 2007 preupgradecheck stsadm command.
SharePoint George has a great post on going through the results of the preupgradecheck output here. This post will give you some great information on tools and techniques for resolving issues that preupgradecheck has found with your 2007 farm.
One of the big challenges I have realized we are facing is resolving missing references to various SharePoint artifacts. I will frequently update this post with the missing SharePoint artifact references that I find in hopes to help out the community when resolving these issues.
Information on resolving issues in preupgradescan by output headings:
Issue : Missing server file or server configuration issues
Finding and remove invalid web part references: http://blog.brainlitter.com/archive/2010/05/18/sharepoint-2010-preupgradecheck-web-part-whack-a-mole.aspx