I thought Branson was against buying into an airline but rather set-up his own (due to the bad experience he had with Virgin Express at the start). Besides the law in the US prevents foreigners from owning more than 49% of an American company.
This is probably just a rumour and won't materialise. Though it would be interesting if it does happen. The VS
company will probably concontrate on launching a LCC before taking such a major decision.