from a machine running the 32-bit version of the management tools plus a local install of outlook 2007:
Export-Mailbox -Identity <username> -PSTFolderPath <example: C:\PST>
(run the Exchange Management Shell as Administrator)

the domain user logged in must have the appropriate exchange privileges and full access permissions to the mailbox being exported.

source:
http://blogs.technet.com/b/exchange/archive/2007/04/13/3401913.aspx