In case you haven’t noticed, Microsoft took a giant leap backwards from their AD management of Exchange. It’s now back to the way it was Pre-2000 (5.5 anyone?) instead of 2000/2003 ish. One of the nice things about 07 is that they gave us a command line interface that is actually pretty nice (once you get the hang of how it all works).
This is how to setup a user with full mailbox rights – you can also choose other options instead of FullAccess.
This also assumes that you know the login/mailbox of the user you want to give permissions to as well as the user you want to take the permissions from.

Open up the Exchange Management Tools
Add-MailboxPermission OF_USER -User TO_USER -AccessRights FullAccess

So in my case it was:
Add-MailboxPermission computeruser -User computeradmin -AccessRights FullAccess
Which gave the computeradmin full access to use the computeruser’s mailbox.