In the new Last.fm widget gallery, I found this add-on for Windows Media Center: LastFMCE. This add-on allows you to tune to your Last.fm radio station and listen to it inside Windows Media Center. You can also use it to search for tags, artists, or a radio URL. By browsing artists or genres, you can use LastFMCE to find a station you love.
Unfortunately, the link on the Last.fm web page is dead, so I found the add-on was still available from SourceForge here. But, before you get all excited, it appears that this is only available for WMC 2005. However, since the code is out there, here is my plea to you: can someone please, please port this to Vista's Windows Media Center? I really want this!