A new Exchange Hotfix is making the news right now.  It's only for Exchange 5.5 servers, but you will want to evaluate whether or not you want to apply the hotfix before you start to have Outlook 2003 users (which launches Oct 21).  It does require a pretty complicated kind of rule to exist, so it should be rare.  From the KB article linked above:

Using the Microsoft Office Outlook 2003 client to modify rules or to import rules into a mailbox that is located on a Microsoft Exchange Server 5.5 computer can result in a server-side rule with a malformed property. The execution of rules in this condition may cause the Microsoft Exchange Information Store service to unexpectedly stop responding.