Oh how i hate licensing questions! :-)
Heres the latest on MOM 2005 which i got from the FAQ on MOM 2005... http://www.microsoft.com/mom/evaluation/faqs/default.mspx#EYE
MOM 2005 licensing model has changed from a per–processor to a server plus managed device basis. In MOM 2005 we need require a MOM server and an OML license.
OML is Operations Management License, which is required for each device managed by MOM 2005. One thing to note is that virtual machines do not require an OML as the MOM 2005 licensing model is based on the physical machine. Also this is regardless of how many management packs are used on the server.
Bottom-line, you need a MOM 2005 Server license per server and an OML per device.
How much is it gonna cost you? The estimated retail price of a MOM 2005 server license is $729 per server (MOM server is not available individually in retail, and is sold with a minimum of 10 OML pack. MOM server plus 10 OML estimated retail price is $6,099).