Earlier on today I came across an advert (within a Twitpic page) enticing users to click through to a web site in order to enter a competition to win an Apple iPhone.
Clicking on the ad takes you to a simple 'spot the difference' quiz on the competition web site.
Clearly not too challenging a quiz. And very forgiving - if incorrect, the user is simply prompted for the right answer.
Anyway, back to the point. Is this a scam? In this particular case, no, I do not believe so. The competition appears to be legitimate, and is hosted by a known company with something of a history in this area.
However, for every legitimate competition out there, there will be numerous others with more sinister data-gathering motives. And it is virtually impossible for typical users to distinguish between them (providing a subtle touch of irony given the 'Spot the Difference' competition theme). The fact that a site looks professional with slick graphics means absolutely nothing.
Users should also be aware that even for the bona fide competitions where there is actually a prize to be won, nothing is free. The 'cost of entry' will be some level of personal data, be it an email address, postal address, telephone number or date of birth.
Note also the checkbox near the bottom (checked by default). Agreeing to be notified of other prize draws actually signs you up for a chargeable alerts service.