I just tried it. Put a splitter on my RR line and plugged in a HD TV. You don't get much, but you do get a few. Fox, NBC, PBS, and a few I'd never watch. Might be good for a guest room that seldom gets used, but I'll stick with Dish. Be converting to their Hopper/Joey system soon. Still cheaper than TWC after the perks run out.
edit: The splitter I used is only rated at 400 hz. If I used a high frequency splitter, would any additional HD Channels come in (like CBS and ABC)?