Just tried Punki's suggestion on Boston-London. The site said 3280 miles. AA gives 3265 to Heathrow (west of the city), 3283 to Gatwick (south). Certainly close enough for planning even though the site presumably figures city-center-to-city-center distances and airlines (at least AA) use airport-to-airport. And the user interface is as painless as he suggests. I've added it to my bookmarks list. Thanks!