Fergus and Ostby are probably busy with Cowboys and Aliens, which Downey has been attached to star in, and which is also set for a summer 2010 release.
IMHO, Iron Man's script was good in spite of the fact it was handled by a cadre of writers, and that extensive improv was allowed, not necessarily because of it. So many cooks usually lead to disaster. I think that it did work is a testament to Marvel, Favreau's and Downey's knowledge of the property and their filter will hopefully provide the same result for the sequel regardless of who pens the script.