I agree with CX747. I don't think BA & UAL will be big operators of the 777-300. Afterall, they are the biggest 747-400 operators.
However I got this from the Airbus site (Sorry I don't have a direct link..their page is in frames so the address doesn't show)
>>Representatives of airlines from around the
>> world are working with Airbus Industrie with >>the aim of launching the new aircraft --
>> designated the A3XX -- in time to start
>> passenger service in the year 2004. These >>carriers include United Airlines, Japan Air >>Lines, Federal Express, Virgin Atlantic, >>Singapore Airlines, Air Canada, British >>Airways, Cathay Pacific Airways, Air France, >All Nippon Airways, and Lufthansa.
You can go to www.airbus.com, click on Aircraft Family, Click on Product Development, then click on the A3XX link.