Can a developer with an app in the market also distribute to un-registered (rooted) devices?
I don't think there's anywhere to cite where Google has said that putting your app on the market is not exclusive, but I'm sure it's allowed, since there are a lot of different alternatives to the Market and you can even download versions of apps available on the market through other means. In certain cases, like Tasker, getting it directly (not through the market) is even encouraged by the author.