The mantra of Fortune's 2012 40 Under 40? Mobilize! All told, this year's list reveals a drive and a determination — indeed, a mobilization — at its core. Yes, it features plenty of Silicon Valley hotshots. But it also includes the CEO of a blue-chip beer brand, an airline chief who just placed the largest order in commercial aviation history, and Warren Buffett’s new lieutenant. Oh, and there's a new No. 1.