Jeremy Toeman has put together a great post of seven do it yourself viral marketing strategies that's worth a read. Strategy #6's explanation of how employees of companies can contribute to product review or blog comments without coming across as shills or creating an astroturfing scandal was particularly interesting:

When you consider the power of discussions and the individual’s ability to create noise, the less opportunity you have for dumb scandals, the better. As an example, at Sling Media I implemented a strict policy that no employee was to add comments about the Slingbox on any blog or review site (such as CNET or Amazon) without disclosing their employment status. Why? Well, look at the Amazon page for the Slingbox Classic - it’s averaging 4.5 stars from 176 reviewers (at the time of writing). If the company can honestly state that none of them are employee-fed, then they gain a heck of a lot more trust than if there’s suspicion about shills.

Engaging people is important. It wins people over because it proves that you're listening. However, as Jeremy explains, it can backfire if you're not open and honest about your relationship to the product or company you're commenting on.

At it's worst, we've seen PR companies like Edelman - who are paid to be experts at this sort of thing - set up fake blogs written to sound like grass roots blogs praising companies like Wal-Mart.

Personally, I don't think Amazon's users would object to product reviews that address the concerns of previous commenters as long as their is disclosure and the post provides value rather than cheerleading. For example, a positive review by an employee could explain that a bug that's been complained about in earlier posts has been fixed, products shipping after a certain date will no longer have that problem, and earlier models can be updated with a patch. That's valuable information to people making a buying decision.