Been looking for some stuff on this and came across a few links I wanted to share.
First of all my buddy Doug posted a blog up on MsExchangeTeam (well done m8) and that gives you the reasons why you should (http://msexchangeteam.com/archive/2006/10/18/429216.aspx)
He also links out to the Exchange Partners page (http://www.microsoft.com/exchange/partners/default.mspx). If you drill down under compliance (http://www.microsoft.com/exchange/partners/2007/compliance.mspx) you get a list of potential products that you can use (didn’t know about this!).
This is quite a good PPT from Kieran McCorry (http://h71028.www7.hp.com/enterprise/downloads/Journaling_Archiving_and_Regulatory_Compliance_with_Exchange_2003.ppt)
What also seems interesting is that instead of having your servers, you can actually outsource this another company. Check out Microsoft Exchange Hosted Archive (http://www.microsoft.com/exchange/services/archive.mspx)
These would be good article, but you have to register and subscribe to see the whole thing :-(
An Exchange 2003 Journaling Primer: http://www.windowsitpro.com/MicrosoftExchangeOutlook/Article/ArticleID/45348/45348.html
Develop an Exchange Compliance StrategyDevelop an Exchange Compliance Strategy: http://www.windowsitpro.com/Windows/Articles/ArticleID/94390/pg/3/3.html
As they say, every day is a school day!