The FTC (Federal Trade Commission) is getting interested in US bloggers who accept freebies. I think this is just the soft side. In the background it’s an attempt to get things organized so the IRS (tax collectors) can report on blogger income and collect a share.
A lot of people are mentioning using Disclosure Policy, the site that sets you up with a disclosure policy once you answer a few questions. I don’t think this is really what you need. But, if you’re worried about it, you have the option.