Companies are trying to use legitimate looking blogs as ads. They are called fake blogs or nicknamed flogs. Sony, Walmart, McDonald's and Coka Cola are large companies who recently had flogs unmasked. Even with the embarrassment of having their flogs unmasked, it will probably not stop the practice because there is so much interest in blogging, and its power as a marketing tool. Also, most companies are not usually aware of each and every tactic that marketing agencies use. Some web users have figured out ways to find them, mostly by using the whois information, this has been countered recently by using third party services such as Domains by Proxy or making the whois information private.
There are plans in the UK to make it illegal for companies to create blogs under false identies. There is no such plan here in the US.