I just stepped into a role where there are multiple sites and the primary site has been transition to Exchange 2010 but the Exchange 2003 servers are still in place.  It's been years since I have dealt with E2k3 and the person who started the transition here is no longer with the company. Here are my questions:

How can I tell what, if any, roles are still on the Exchange 2003 servers?
When is it safe to shut down / uninstall the Exchange 2003 Mailbox servers?

Is there any tools out there that can look at your messaging environment and let you know what servers are safe to shut down?

Actually, it's pretty simple. Make sure that you don't have any mailboxes left on your old server. Also, ensure that you have all your address lists and email address policies upgraded to a new version. Check if there are public folder replicas left there, if they are, remove them. Then try to run uninstall procedure for Exchange 2003. It will not let you remove Exchange 2003 if there's anything critical left there.

Also read this article: http://technet.microsoft.com/en-us/library/gg576862(v=exchg.141).aspx