We currenly have Exchange 2003 and Outlook 2003. We're planning on moving to Exchange 2007, but keeping Outlook 2003. Do we need to purchase Exchange 2007 CALs?

Yes, but you should discuss these matters with your preferred Microsoft reseller. There are so many options to chose from. Exchange Server 2007 Licensinghttp://www.microsoft.com/exchange/2007/howtobuy/default.mspx#EKCMCTS: Messaging | MCSE: S+M | Small Business Specialist