Businessweek has a story about an astroturfing debacle where Wal-Mart was (indirectly) paying bloggers/journalists to say good things about them. This seems to be happening more and more. And it will, too.
I think the reason for this is a simplistic mind-set:
1) Big company finds bloggers saying bad things about said company
2) Big company pays money to other bloggers to say good things about the company