I had always assumed that to recover a a mailbox when the original MsExchangeGUID attached to the AD account was lost would require a replica AD and Exchange 2003 server to be built.
This is just not the case and I've already restored several mailboxes by relinking them to different AD accounts in entirely seperate forests.
the key to it is LegacyDN.exe,a super tool which can be downloaded from www.microsoft.com/exchange. Don't ever run it in your production environment though..
Once in possesion of this, all you need is the .edb and .stm files.
I have an entire Virtual Server 2005 environment now for this. Create a new AD (call it whatever you want), extend it and install Exchage 2003. Call the Exchange org whatever you want, and the server itself can be whatever.
Once all is installed, I'd copy the VS2005 disks so you always have a ready environment for re-testing.
Running legacyDN.exe, you can then alter, the Orgname, the AG name, and the all importany LegacyDN name to match where the original DB came from. Once done, create a new store, but don't start it. Place the origianl DB's files in the location of the new store. Start it, run mailbox cleanup agent, then relink to any AD account you've created in your new directory.
Export with exmerge and job done.
I know RSG's save us in almost all cases, but exmerge will not see data when the msExchangeGUID cannot be matched to a AD account.
With the Virtual Server disk images in place, I can recover any mailbox in about 10 minutes these days.
Long live the illusion of identical environments for restores required, NOT ! If you want the full step by step guide, drop me an email.