This is a bit of a strange area as the camera you choose can have a large affect on whether an image of your will get published.
As Arkady has said, if a magazine likes your picture then you'll have no problems at all with anything of reasonable file size. However if you go via an image library or agency you may well come across some model predjudice. I know for a fact that my wifes 10d is more than capable of the quality for an a4 mag image and if it's spot on may even go to a3. Most libraries wouldn't even look at anything that's not shot a "pro" camera though.
