Not that hard, just a pain
You can get hired at a airline without a US passport. There are plenty of "foreigners" at all types of airlines. It's a pain in the butt however, because everytime you go to training, the airline has to do a thorough background check on you again because it is required now since 9-11. Since those guys (hijackers) were taking flying lessons on a simulator before they did something stupid. Now, you as a law abiding person are paying the price for their actions.
What I mean to say is that there is a lot more paperwork involved since you aren't a US citizen. Some airlines don't like to deal with the hassle of all the paperwork involved, so yes it may be a little more difficult for you to get hired here, but it's definately not impossible. Get your app. out there and see who bites. Enjoy the paperwork!