One of the things that makes word-of-mouth effective is that it's perceived to be based on authentic and commercially disinterested opinion. So, as soon as you sense that I stand to profit from you acting on my recommendation, you become more wary of my recommendation and less likely to act on it. (I've written about this in the context of blog marketing on my DJ Alchemi blog.)
In the music area, a couple of initiatives have started, with mixed receptions. Weedshare has been running a scheme for several years now, which encourages you to share digital music files with your friends by email, instant messenger or your website. They can listen to each file three times for free, but then have to pay to enable unlimited plays. If they do pay, you get a commission. It seems like a transparent model, and fairly simple in broad outline (though I think it gets complex in the details). The Digital Music News blog and associated comments give a mixture of opinions. It's fair to say that Weedshare hasn't exactly taken off and hit the mainstream, but that's possibly because (a) it has no major label music, (b) it uses Windows Media files (incompatible with iPods) with DRM, and (c) on the receiving end, illegal peer-to-peer sharing is more attractive, because there's no payment.