You know, I am really torn on the idea of WotC doing 4E updates for Dark Sun, Ravenloft and Spelljammer.
I mean, on the one hand, SWEET!!!!! Dark Sun!!! Spelljammer!!! And lets face it, when WotC decides to do a campaign setting, they do it really, really well.
But on the other hand, one setting per year means we could be waiting a long, long time before those settings get any love. A license to a 3PP would see a much faster release cycle.
And now for some cheap (and easy!) speculation about the settings:
To avoid duplication, Dark Sun can not be released until after the Psionic power source has been fully developed (PHB II, but also "Psychic Power" or whatever that splat is called). The Dark Sun DM's guide will have rules to support generic "Desert-based Adventures" which, when combined with some DDI articles, will allow you to run Al-Quadim.
Spelljammer (or possibly, a Spelljammer-Planescape combo: Planejammer!) can not be released until (1) after Manual of the Planes II develops the concept of "multiple worlds, much like our own", and (2) PHB III presents rules for firearms and powder-based artillery. Alternatively, Spelljammer will be the source of firearms rules.
Ravenloft can not be released until after the Shadow power source has been fully developed. It will introduce the Demonic (Evil Primal) power source. Shadar Kai will be given a realm, and purists will hate that.