I think this is much more common than you might expect. I don't like the idea of asking for compensation for a favorable mention, however is it all that different from when a company gives away their wares to someone unsolicited? I suspect that most people in that position will have their opinion slightly altered by the act of getting stuff for free. Many bloggers get stuff sent to them by various companies, some keep the items, others give it away to the readers.
So is asking to keep the item, or asking for an item all that much different? I guess its more overt from the author that they are after something, but if the author still maintains an objective viewpoint what harm is really done?
Can you really trust the product opinion of someone who hasn't put out their hard earned cash to acquire it? Amazon reviews in my eyes are much more powerful than something like Consumer Reports. What about you?