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: 
[GFI MailArchiver](http://www.gfi.com/mailarchiver/) is the 3rd party product I see recommended a lot for this purpose.

And while this won't be of any help now, many of the features in Exchange Server 2010 will focus on easing [archiving and retention](http://www.microsoft.com/exchange/2010/en/us/Archiving-and-Retention.aspx).
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.