Use the Set-AdServerSettings cmdlet before your Get-MailboxDatabase call, or put it in your $Profile:
Set-AdServerSettings -ViewEntireForest $true
While I'm at it, if you mearly want to get a count of mailboxes, you can do it like this:
(Get-Mailbox -Database <foo>).Count
Karl
(Set-AdServerSettings is on page 332 of my book) ;)
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