The US has a number of circumstances where "government issued ID" is demanded and an "out of state" drivers license isn't good enough, escalating immediately to passport. This includes things like checking into a hotel, picking up your badge at a conference and (I am not making this up) buying medication for a head cold. Mark mentioned getting into a bar which is not an issue for me, but the point is that people want to see your ID rather more than I (a Canadian) expect them to, and if you're not local, they want to see a passport. If an internal domestic flight might not mind you using your UK drivers license, that wouldn't mean that once you got into the States you could ship your passport off for visa-getting purposes or whatnot, because chances are you'll need it pretty regularly while you're there.