For UA, they just let you check your luggage and you can check in at the regular counter, gate or in the Red Carpet Club if you have a membership.

One warning, last month I was in Las Vegas and a guy had used curbside check in for his luggage. By the time he got to the gate to get his boarding passes, there weren't any confirmed seats left for him since the plane was very overbooked. His friends who had decided to go to the regular counter got confirmed seat assignments with no problem.