A friend of mine who is a Canadian citizen and has applied for a green card here in the US, went home for Christmas. She got into Canada, but they wouldn't let her out. Seems you can't leave the US once you have applied for the green card until you actually get it. And if you want to leave, you have to have permission in advance of your travel.
Shrugs.....well that is what she went through anyway. They finally let her back into the US, but she can't leave again until her card comes through.