Thanks for the replies, just a small clarification to my question - I use Entourage to connect to an Exchange server at work, so I was curious whether Spotlight would be able to index the emails. I have a rule set up which copies all emails from the Exchange server to a local inbox in Entourage, so as long as Entourage supports Spotlight or vice versa then it should be OK. May have to wait for this to happen in an update I suppose.
I only access my IMAP personal mail using Apple Mail, so I'm hoping that Spotlight will treat IMAP mail on the server the same as POP mail which is downloaded to a local mailbox.