It is simple using the exchange management shell to export mailboxes to pst in 2007 and 2010.

Firstly you can export mailboxes individually
Export-Mailbox –Identity ‘alias’ –PSTFolderPath

Secondly you can export an entire database
Get-Mailbox –Database | Import-Mailbox -PSTFolderPath c:\PSTs

Thirdly you can export by OU, handy if you have an OLD or ARCHIVE User group in OU you want to archive
Get-Mailbox -OrganizationalUnit | Export-Mailbox -PSTFolderPath c:\PSTs

Important to note, as with exmerge in 2003, exporting data from a mailbox to a PST file does not delete the content in the source mailbox. If you want to delete the content you have to include -DeleteContent or -DeleteAssociatedMessagesmessages.