As someone already said, if you have a device that connects to the web and it has a storage device then there is a risk, no matter how small, that a virus can be obtained. Last time I looked, all of the net-gen consoles fit that description.
I would argue that the Xbox is the least likely to contract a virus as it is the only console to have a very closed network. By that I mean that you are not going to be web browsing, down loading porn, checking you e-mail, playing with PC players, etc. The Xbox network is just that, a network composed of Xbox\'s (and Xbox certified servers). Because of that, your exposure to viruses is reduced to a level that no other console can claim a right to.