No MP credit, tho
I flew on ExpressJet from IAD to CHS on 12 Mar, UA5205. The miles for that leg didn't show up in my summary. When I tried to request credit for them online, the web site said:
The operating airline is not a Mileage Plus partner. Therefore, this flight is not eligible for credit.
It was marketed, sold, and operated as a "United Express" flight. I talked to an MP rep, and he repeated what the web said, that United Express flights operated by ExpressJet do not count toward any mileage credit. He said that there were no future plans to give mileage credit for ExpressJet flights.
He also couldn't provide a list of earning United Express partners. I got that from a regular customer rep: Colgan, GoJet, Skyways, Mesa, Shuttle America, Trans States, and Chataqua.
So, at this point, buyer beware. If you fly a United flight operated by ExpressJet, you get no miles credit.