One of the challenges the MCE Product Group faced when the product was first created was how they would distribute the product to customers. Would it be through major OEMs, through the broader System Builder Channel or directly to customers as boxed product at places like Best Buy. As you know with the first two versions, the decision was to stick with major OEMs. Why? Well, one of the things to understand about MCE is its dependence on specific hardware (video card and TV tuner, for example...) and specific versions of drivers for that hardware. Using unsupported hardware or even inappropriate versions of drivers for supported hardware would degrade the end user's experience.
With Media Center 2005, distribution was opened to the System Builder Channel in addition to the major OEMs. I presented the technology to a group of System Builders in Indianapolis last week and they were all very excited about the new offerrings they could provide their customers. If you're one of these System Builder folks or a developer building a system for test and development purposes, it's important to understand what's compatible and what isn't. There's a list of known compatible products at http://www.microsoft.com/windowsxp/mediacenter/partners/dfw/partnerlisting.mspx. Check it out and save yourself a lot of time and trouble...
Here's a nice JPG of how all the pieces go together, by the way - http://www.roudybob.net/images/ConnectingMCE2005.jpg.