Kos is mostly right. Preventing blogs with an agenda, fake blogs, paid bloggers, etc. is just not doable. And why would you want to? The whole premise behind the Marketplace of Ideas theory is that the cream rises to the top, no?
The only thing I think the blogosphere (and the Internet in general) needs to get a handle on is anonymity. My opinion on anonymity differs significantly from that of the mainstream. I believe that if something is worth saying publicly, the idea should carry your identity with it. If it doesn't, anonymity becomes a license for people to sling all sorts of shit they know to be untrue. This is one of the reasons why I insisted nearly a dozen years ago that the then-fledgling Sound Observer have a strict "no anonymous letters" policy.
It's way to easy to set up an anonymous blog. It's way too easy to send anonymous e-mail. It's way too easy to participate in distributed conversations under a pseudonym. I don't wish to see any form of prior restraint (even if it were possible), but I would like to see identities attached to ideas, especially when we're talking about political speech. It's a notion that has impact on believability, credibility and, perhaps most importantly, accountability.
But in the absence of such things, I say let the Wild West atmosphere permeate everything it touches. Let the cream of the idea crop rise to the top.