Look at this service. It's basically a system where you pay to have bloggers claim to have reviewed your product or service. 'PayPerPost' is, at least, I suppose, an honest name for it. Hilariously, you can choose whether to allow only positive testimonials or positive and negative. Bloggers are now required to disclose that they're being bribed, though they don't have to do so on each post and they didn't have to when the programme started.
Rather absurdly, the founder says that it is ethically equivalent to having banner ads.
Lots of coverage here.
I really hope this sort of thing doesn't become common; right now, blogs are a relatively decent place to get book reviews and things.