This may be old news, but I got this email from Orbitz...
Travel to London just got easier with our new nonstop service from Philadelphia to London's Heathrow International airport. Whether you're flying to London for business or pleasure, now you've got more choices. Make the convenient commute from Heathrow to the financial district, or take the Tube to Tower Bridge with ease. You can also make a connection with a Star Alliance™ partner airline to almost anywhere in the world. Whether you're on holiday or have an important meeting, get there with US. Service starts on March 29, 2008, so plan ahead and save.
Sample Round-trip FaresFares include service fee
Los Angeles-London $496
Los Angeles-London $433
New York City-London $389
San Francisco-London $553