I just went and saw THOR today, and I can definitely confirm the Hawkeye sightings!
Apparently in the movie Hawkeye is working for SHIELD as some sort of special agent (under Coulson). And in the scene where Thor tries to break in to get Mjolner, Coulson says he needs an eye in the sky, or something like that, preferably with a gun. So Hawkeye being the agent he charges to do this with, runs to the weapons truck, looks over an array of miscellaneous guns, than grabs a bow of the wall, before jumping into this lift thing that pulls him up above the room where the hammer is. Hawkeye then continues to communicate with Coulsen as Thor tries to pull the hammer from the rock.
I'd say that's definitely more than a cameo.
Stan Lee in a pickup truck with one line is a cameo.
I'd call it a special appearance.
PS I hate the note at the end of the movie. Sure lets just spoil it for every one huh?