Yes, you can earn miles on United Express flights.
In May, I flew to FAR on United Express, and received miles for the flights. I had to mail in the boarding passes, as my AP number was not in the computer(again).
http://www.aircanada.ca/aeroplan/partners/united.html
The accumulating miles section of Aeroplan states you can earn miles on United Express, but does not mention Skywest.
[This message has been edited by Flying Buffalo (edited 06-17-2001).]