3rd party publishers also do not know if they will be able to use everything in the new 4th edition players handbook, DMG, and monster manual and what OGL requirements will be. Sure in 3rd edition everything but a select group on monsters out of the monster manual was fair game (and you could always create new monsters to fill in the holes), but I no longer think that the "focus your 3rd party products to sell WotC core books" business model is still in effect and it is possible that some of the new races and class may not be available to everyone. I would bet money that some of the new monsters will not be, just like in 3rd edition.
Most likely though I think most everything will be open source for the 3rd party publishers but they will be required to help market the digital initiative and WotC miniatures. Wizards has been devoting its resources to creating the digital initiative and in a lot of ways using the core books to sell it (unlock extra content on the web, auto updates to rules as they become availalbe, and most importantly stream lining the rules to make it easier to computer code, just like they have been with Magic the gathering). I can not see WotC pumping so much money and resources into something if it is not going to be their main source of revenue in the future. Think about it, it is a much better business model to let Paizo sell a Pathfinder AP module to one DM in a play group ($20 a month), then have everyone in the play group pay to play on the digital initiative (5-6 players, 9.99 each per month). In fact most AP modules take longer than one month to complete for most gaming groups, even better for WotC. You do not have to like the model, but lets face it we all know there are a lot more players out there than book sales indicate and if you could tap that market, even if each player only had to pay 2.99 a month, WotC would be making a tidy sum in the long haul. The question is will 3rd party publishers still find a niche for profit in 4th edition remains to be seen and that is why I would not hire a freelancer to do anything for me till a see the new rules of the marketing game.