I suggest some of you folks read the several books out on Branson, including his biography. Then, you will have some idea what this is all about.
There have been plenty of US start-ups over the years who have taken years to get off the ground. It makes economic sense to wait for the "right" moment to launch. Right now is not the time with fuel being the way it is and the top 5 carriers in bankruptcy protection - hardly conducive to good business practices and you don't need to be an MBA to figure out that one.
Branson is a canny businessman. How else do you think he's made out with some 200, yes, 200 companies under the Virgin Brand. Not all have worked out as planned, some have been PR disasters but he learns from his mistakes and most of his ventures are successes.
As someone mentioned, he will not have a controlling interest in a US company but he is not keen on any merger with a current US airline - those are just rumors! Why should a new company merge with an already existing one and bring upon itself the complexities of that union. Again, doesn'y make much sense. Read the book(s). They are more accurate than any rumors among F/As and buddu buddies.