EVEN IN THE MEGABUCKS world of mobile telephony, this one is big. Vodafone, Britain's largest and most successful cellular-phone company, is paying more than $60 billion for AirTouch, a leading U.S. concern. That trumps a bid by rival Bell Atlantic and makes the deal one of the biggest in corporate history. The new outfit will be the only truly global wireless operator--with coverage on five continents--and gives it the scale to challenge the giants among the fixed-line operators such as AT&T. The pool of potential customers is vast. Vodafone's chief executive, Chris Gent, has reportedly forecast that every other person in the developed world will soon have a mobile phone.