A few main points:
- Testing, testing, testing! Even if you have gone through dozens of migrations from 2007 to 2010, each farm is different and you should apply the proper due diligence to test the migrations prior to performing the actual upgrade.
- The MOSS 2007 farm must be SP2 with the latest CU paks with the CU from December being recommended.
- SP2+ on MOSS 2007 makes SharePoint aware of the database in read-only mode. Therefore, you can make the content databases of your MOSS 2007 farm read-only and SharePoint will automatically hide any Edit, Add, or Delete operations from its menus.
- Making your databases read-only is a good way to migrate the data since users cannot change anything while you are upgrading. However, the content databases that are being migrated cannot be read-only so you will need to make a copy before attempting DB-Attach operations.
- The migration check in MOSS 2007 is the preupgradecheck in stsadm.
- The migration check for SharePoint 2010 is the PowerShell command TEST-SPContentDatabase
- In-Place migration methods seem to handle most farms, however, it is not recommended as if anything goes wrong, there is no roll back. Your MOSS 2007 farm will be down until all issues can be resolved.