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.
If you are looking for ongoing archiving of email for legal reasons etc. you might consider google postini. See [here](http://www.google.com/postini/).