Not to cause a ruckus... but having seen this commercial a few more times, I'm not sure whether there's a body double and if so, where she's used. The long slow establishing tilt up is the clearest display of the bust to waist ratio (and quite remarkable for mainstream TV... must be some true tit-fiends at Michelob's ad agency). However, there's at least one underwater shot (looking straight up) and one end shot that also features the girl with the impressive rack. The only face shot that looked different from the rest to me was the one while swimming (breast or butterfly stroke, I'm not sure) but I think that looked different because her hair was darker (wet) and slicked back.
Anyway, this is certainly worth notice if for no other reason than the genuine D cups on an otherwise slender athletic frame.