We're moving from Exchange Server 5.5 to Exchange Server 2003 and want to consolidate our existing servers on a lesser number of clustered mailbox servers. Can we make the first Exchange 2003 server we install in our organization a cluster?

You can certainly have Exchange 2003 clusters in a mixed-mode Exchange 2003 (or Exchange Server 2000) and Exchange 5.5 organization, but the first Exchange 2003 server you install can't be a cluster. The Site Replication Service (SRS) is required on the first Exchange 2000 or later server that you add to a mixed- mode organization, and the SRS won't run in a cluster. You need to install one unclustered Exchange 2003 server first, then add the clustered server. After you've decommissioned all your Exchange 5.5 servers, you can remove the SRS (by following the procedure in the Microsoft article "How to remove the first Exchange Server 2003 computer from the administrative group" at http://support.microsoft.com/?kbid=822931) and recycle the former SRS server into a cluster node.