Some things to keep in mind during the migration:

1. Tons of transaction logs will be generated and won't be wiped until a backup happens...make sure you have space on your transaction log disks.
2. Make sure your Mailbox Server has plenty of physical resources available to use during the move...RAM is a specific concern.  Always overshoot your estimates on how much RAM you need.
3. Error reporting in the shell isn't as intuitive as you'll get using the EMC (Exchange Management Console)

My specific environment was moving from a single backend 2003 server to a single 2007 mailbox server.  The cmdlet that is posted can be used to move a mailbox database/storagegroup from one physical server to a another database/storagegroup on a different physical server.  I'll go through what each section of the cmdlet means.

[sourcecode language="plain"]Get-Mailbox –Database “2003ServerName\StorageGroupName\DatabaseName” | Move-Mailbox –TargetDatabase “2007ServerName\StorageGroup\Database” –MaxThreads 10[/sourcecode]

The command above should be executed in powershell on your 2007 Mailbox Database Server.  The Get-Mailbox -Database command will tell Exchange where the mailboxes you are moving are coming from.   You then Move-Mailbox to a target database on the 2007 Mailbox Server.  The last portion of the command tells the command to increase the max threads of the command from 4 to 10.  Please make sure that you substitute in the server names, storage group names, and database names appropriately.

Remember that this will only move one database at a time...if you are like me, you like to take small steps in the process and verify that things look good after each small step.  Hopefully, this helps your migration take a little less time.