How to copy mailboxes from Exchange 2003 to Exchange 2007 across forests

Nope-- you can't just move the database around. It's too bad, but that's just the way the product is designed / implemented.

Your best bet is to ExMerge out the data from the source server, and import it into the Exchange 2007 server.

More background on ExMerge: http://support.microsoft.com/kb/174197

Bear in mind that replies to the imported messages may fail and modifications to old calendar items may not work right because the X500 addresses from the original Exchange install won't be assigned to the mailboxes in the new Exchange install. There are some details about that problem here: http://blogs.technet.com/sbs/archive/2009/05/21/cannot-reply-to-old-emails-or-modify-old-calendar-items-after-pst-mail-migration.aspx

Basically, you need the legacyExchangeDN from the old Exchange install to be added to the new recipients as an X500 format proxyAddress entry. It's not too bad to do, once you know what to do.

I would suggest using the commandlet Export-Mailbox, as you described. There is an article on the MS Exchange Team Blog that goes over its usage and discusses the permissions needed. (There is a newer post as well that is more clear)

Since I have found their post to be somewhat vague on setting the correct permissions, I prefer the instructions in an alternate post, which show how to give yourself permissions on all accounts or only on an individual account (my preferred method when I only have to export a few accounts). Basically, before running the export command, you run:

Add-MailboxPermission –Identitiy “MailAlias” –User Username –AccessRight FullAccess

where "MailAlias" is the account you are trying to export, and Username is your account (or the account you will be running mailbox-export as). After that, export away:

Export-Mailbox –Identity <mailboxUser> -PSTFolderPath <pathToSavePST>

Lastly, to be clean, I like to remove the permissions and return things their original state, so:

Remove-MailboxPermission –Identitiy “MailAlias” –User Username –AccessRight FullAccess

After that, you are all set, you can take those PST's over to your new server, and import them. (You will probably need to grant yourself FullAccess to the new mailbox in order to run the import)

As an aside, the reason you need to run the pst export on a 32-bit machine is that the export has a dependency on Outlook, which is currently only 32-bit (this is described in the second posting from the MS Exchange Blog). I recall somewhere that MS was planning on correcting this limitation with the release of Office 2010.