Do many of you remember those cheap East Coast USA to LHR fares back a couple of years ago? You could fly for about $250 RT including all taxes in off season, Boston, Washington, New York to London Heathrow on various competitive airlines.
Now, you can't pass through LHR without shelling out $250 in taxes. The weak dollar, the high security taxes, London fares are about $450.
Going to Stansted, Ryanair et all keep their fares low by flying out of secondary London airports - Luton and Stansted to be precise. I imagine that some of the high tax costs would be alleviated if the US majors flew direct to Stansted. The new all business airlines nowadays (i.e. MaxJet) are flying JFK - STN.
I think that if some of the US airlines started flights to Stansted it would not be such a bad idea, if it would keep fares inclusive of tax down.
Also, for young and budget travelers doing Europe from the US, I think there is a potential market. For many/most travellers London is still among the cheapest direct destinations from the US. Getting in early to STN and then catching a cheap flight to Spain, Italy, Germany, or wherever via EasyJet or Ryanair may not be bad.
Of course, the later part would probably not be a thinking goal of the US airlines, as they would likely a person transferred to one of the respective alliance partners, but still...
Every time I went to Stansted back in my days, I am amazed how many Americans are there transferring somewhere for cheap -

My brother, for example, last summer visted a friend to Germany. He was to originally book a flight on LH BOS-MUC. It was about $1200, which I though was kind of a lot even for the peak summer season. The fare to LHR was about $700 with taxes, and then STN to MUC direct was $100 return. Worked out perfectly and much cheaper...