Reader Nick just emailed saying that, for no reason, a Rayman Origins demo had suddenly appeared in his Steam games library. Not a store listing; it's sitting in his installed games section.
So I fired up Steam and what do you know, it's there for me as well.
There are no actual files installed on your hard drive upon first noticing it; clicking "PLAY" triggers a "content conversion" screen before it begins downloading the demo.
Making things annoying if you have OCD is that you can't remove the game. Clicking "uninstall" will remove the code if you've downloaded it, but the actual listing will remain in your library.
Either this is a strange little mistake, or it's a very unwelcome form of advertising on the part of both Ubisoft and Steam. We've contacted Valve to see which it is.