This is a superb rumour, and it really does look like it could be true as it's straight from John Travolta himself. From Cool Cucumber through Coming Soon:

John Travolta was on The Big Breaky here on TV this morning. He did some stuff for Live-8. Talked mainly about that, but also some of his movies.

He's just done a movie with James Gandolfini called "Lonely Hearts" and he hinted that he may be playing 'A man with a mighty big hat' in a "Dallas" movie. I guess he just confirmed rumors that he's playing JR in the Dallas flick? They didn't push for anymore, just made an "ooohh" sound.

Awww, he was obviously willing to talk about it, why couldn't they have pushed him? Sounds like it could either be a done deal or he's drumming up support to get the part, but honestly, if Travolta wants to play a part in your movie you'd just say yes wouldn't you? No need to drum any support up, so I think he's in. It also suggests that it's going to be a pretty big budget, however it doesn't tell us if the script is going to be any good. Travolta has accepted a fair range of good and bad roles. Is he suited to don the hat of J.R. Ewing?