Are There Any Airlines That Allow Dogs Under 10 Pounds In The Cabin On Transatlantic Flights?

The problem with this question is that the only sure way to answer it is to phone up the airlines and ask.  If you find someone who says yes, then get them to put it in writing.  You want that letter to brandish for when you get to the gate, just in case the staff there don't know what's up.  Moreover, you need to check that the airport itself is okay about animals going through security checks.  Phone BAA, probably.

That said, these airlines are supposed to be good bets, having at least let small dogs into the cabin as hand luggage, in the past.:

British Airways
KLM
Lufthansa

It's worth also asking United, American Airlines and Virgin.

I suspect there's a whole 'nother question in how to best travel with your pet.  In a box, with pet passport and every scrap of relevant paperwork you can think of, and probably sedated.  Your vet should be able to advise.  Feed lightly before the trip to minmise toileting needs.