I'm trying the new iTunes 11.1. You can create your own iTunes Radio stations from a single song and then add song titles to the list of "play more like this" so that the station plays more of the same kind of music, guided by the entire list.
For example, I created an "endless" version of GTA IV's JNR station by adding all of the songs from GTA IV's JNR to the list. So now it should play more of the same kind of jazz based on being similar to all of Rockstar's selections. I even named the station Jazz Nation Radio 108.5.
EDIT: should have mentioned, stations can be shared. Here you go: