If they are ads on Myspace probably not, when it comes to those ads you have to sign up with certain companies and buy a minimum of whichever products they advertise to qualify. But if your talking about the iPhone Blog contest, all you have to do is post 20 posts in the iPhone forum before July 11th to be entered to win an iPhone 3G. And they can't be filler posts. (meaning just random posts that have nothing to do with the iPhone, advertisements, or reposting the same thing. Stuff like that)
Yeah, I had a friend of mine try that "Win a free XBOX 360" a few months ago... It ended up being a total scam and now he's dealing with identity theft issues like you wouldn't believe. Best bet it to stay away unless you're familiar with the site that's offering it the giveaway.