Advertising Messages—How Many Do You See A Day?

I was catching up on some reading today and came across an oft-quoted stat that made me think twice. You've probably seen it before—the number of advertising messages every American is exposed to every day. So what is it? In their book, Under the Radar, Kirshenbaum and Bond peg the number at 1500. But a quick google search pegs the number at:

400-600 a day?
40,000 a year (109 a day)?
2000 a day?
Millions a year (>5479 a day)?
1500-3000 a day? Here too.
2500 a day?
1500 a day?
4,000 a day?

So what is it? Could it really be 5479 a day? That's 1 ad every 12 seconds (assuming you sleep just 7 hours a night). If the number is 3000, that's still almost 3 messages every minute. Anyone seen any research that substantiates these claims?