Contributed By: Cliff Hobbs [MVP ConfigMgr/ SMS]
No. If you ONLY plan to install a server as an ConfigMgr Secondary Site you do not need to buy a ConfigMgr license in the same way as you do for a Primary Site as detailed in the "Secondary Sites" Section of the "Understanding Configuration Manager Sites" article on the ConfigMgr TechCenter:

"The advantages of using secondary sites are that they require no additional Configuration Manager 2007 server license and do not require the overhead of maintaining an additional database."

However, if you want to manage the Secondary as a ConfigMgr Client you WILL need to buy a "Management License for Managed Clients" in the same way as you would any machine you want to manage as a ConfigMgr Client.

For further information on ConfigMgr Pricing and Licensing see the System Center Configuration Manager 2007 Pricing and Licensing page.