Get Paid to Blog Pay Per Post Ethics

Several weeks ago, a mom-blogger was outraged when a marketing company offered to pay her if she wrote nice things about a big name brand that was having an image problem. But is this really different then offering a blogger a free product in return for a review? Most people would agree that a good review requires an honest opinion, but if you get paid to blog does that necessarily preclude an honest opinion?
If the blogger does actually agree with the positive statements they are asked to write, then is it wrong to write an honest opinion and get paid? A case in point is Kmart, who announced that they are looking for gamer bloggers to send to the E3 convention. It is an all expenses paid trip, and all they ask in return is that the bloggers write about the experience and post links to Kmart's gamer blog.