Most of what you hear/read is a smoke screen. Yes, Branson has been looking at various US carriers but he has never made a secret that he wishes to go alone (he has the resources to do this)and do this from scratch. Frankly, the vast majority of US carriers have little to offer him. Investors are lining up at his door, so I think he will be able to pick and choose which route he wants to take.
Why on earth would he want to take on someone else's debt and problems when he could easily start from scratch. Having said that, the US is NOT the same as Australia where it was relatively easy to set up Virgin Blue upon the demise of Ansett. Take a look at the numer of diverse businesses he runs globally. I think you can count on one hand how many have failed or don't make a profit.
They already have a senior management team set up in the US and as a shrewd businessman, he is bound to look at all viable options. However, my bet is...expect a few surprises down the road. This is one guy who knows how to run an airline/business and surrounds himself with talented people, not golfing buddies.
All of the cities mentioned are major US international gateways. And yes, VS has flights into all of them, at least one daily, if not more. Heck, they even fly into LAS 3 times a week.