No purchase required
I did some of the very first online sweepstakes (starting in 1990) so I've seen a bunch, but this is a pretty good story. Pepsi finds a loophole in the rules: Apple - Pepsi - Offical Rules
It turns out that you must offer a "no purchase necessary" option if you want to avoid running a lottery. The Pepsi iTunes promotion offers you a 1 in 3 chance of winning a 99 cent song for free. To get your entry without paying, you must send them a SASE envelope (that means 37 cents plus 37 cents) which means 74 cents for a 33% chance to win a buck.
Wonder how many they get?