If you are trying to uninstall Exchange Server 2010 and you receive an error message telling you that you need to delete or move the Arbitration Mailbox you have to option.
Option one is to delete it by running the following cmdlet from EMS
Get-Mailbox -Arbitration -Database DB101 | Remove-Mailbox -Arbitration -RemoveLastArbitrationMailboxAllowed
If you have another Exchange Server you can move it:
Get-Mailbox -Arbitration -Database DB101 | New-MoveRequest -TargetDatabase db2