I found an article in PCWorld this week that talks about a new Windows Media Center computer being produced by Shuttle with Always Ready technology. Always Ready is sort of a sub-sleep state that allows Media Center to go into a deep hibernation, which is useful to conserve energy and reduce noise. But the machine remains available to do things like record TV programs, and it is able to start back up from the sub-sleep state very quickly. For example, the Shuttle machine in the article could start up in less than 5 seconds.
There is also a slide deck from WinHec 2004 that introduces and explains Always Ready from a theoretical standpoint that I found pretty interesting.