It's not XP Pro either. Media Center should now be thought of as another version of XP just like Home and Pro. It's closer to Pro than Home, the major difference is that Media Center 2005 does not support connecting to a domain. The reason for this is that Extender devices use Fast User Switching technology to work and that functionality is not available on a domain.
If you already have Media Center 2004 connected to a domain, don't worry, when you upgrade to 2005 you'll keep the domain join functionality, but you won't be able to use an Extender device.