I think that cover credit is very important; I also am a "Danceyist" (did I just invent a new term?) where it applies to pricing of product and freelance compensation.
One thing that seems a likely outgrowth of the D20 market is that smart consumers will start paying much more attention to the names on the cover. I think we'll see the development of "star" authors, who will be recognizable names to the general RPG public, creating their own brand, essentially.
People will be able to predict the quality of a product based on the authors name, much like we can predict the quality of a film based on if it was directed by a Cameron, a Raimi, a Lucas, or Scorcese. We'll also have a good idea if the product will fit in our game as we get more familiar with the author's style, and associate it with the author, and not the publisher or the rule set. Just like we know what a Scorcese film is like, regardless of whether MGM or Paramount releases it.
We've already seen it with Monte Cook, obviously Gary Gygax falls in this category, and there's a whole stream of quality guys who keep getting good reviews and putting out good product who could become stars: Mike Mearls, Kevin Kulp, and so on. Sean K. Reynolds will be one, if and when he puts out some freelance D20 products.
Eventually, those "star" performers will be able to pull down larger dollar amounts, or will form their own boutique companies (like Monte did with Malhavoc) and gain more control over their product, possibly also leading to more earnings.
In order to do this, those people have to market themselves as well as their product, to a certain extent.