Paid Links Shouldn't Count? What Exactly Is Paid?

We've had a debate on our SEW Forums recently (see Matt Cutts Comments On Reputable Sites & Link Selling) over whether publishers should tag paid links so search engines won't count them. Part of that covered the issue of links where payment isn't so readily visible. Help Us Build a Link Farm, Get an iPod Nano? from Traffick is a case in point. Want to win an iPod Nano from them? Just post some links to them. It's an example of how what may seem to be a clear-cut, black-and-white issue isn't. Those who forthrightly pay for links potentially might not get those links counted, but do a savvy rub-our-back giveaway and that might fly under the radar?