The guardian has a technology article about a UK study on hypersonic aviation that concluded that producing a plane fueled by liquid hydrogen could feasibly transport commercial passengers on long distances in much shorter times than current planes. Provided the hydrogen is created without using hydrocarbons (not easy currently but potentially doable in the future), the flight will be pollution-low, as hydrogen burns to form plain old water, although as the correction to the article mentions, Nitrogen oxide byproducts would still need to be contained.
There are issues though, before we can hop on a sub-orbital or hypersonic flight. Like the Virgin Galactic project, there are concerns about how well us puny humans can cope up there in high-atmosphere at very fast speeds. And, like Paul said earlier today, the future is expensive, including that of flight – will people be willing to pay many times more for such a ticket? Incidentally, isn’t it neat that the design looks and behaves just like the ‘Fireflash’ hypersonic airliner out of Thunderbirds? Supermarionation is the future!