Question: I am familiar with archiving messages on the client side to .pst's... Are there good methods of archiving messages from multiple mailboxes on the server side?

Answer: 
I recommend Barracuda's [Message Archiver](http://www.barracudanetworks.com/ns/products/archiver-features.php). It keeps a complete archive of all mail. It can also trim attachments from exchange and host them itself. It provides a link in the original message to the trimmed attachments.
To archive a copy of every message, create a journaling mailbox on the server