Contrary to what the prior two answers said, I did mentioned my low GPA (below 3.0) during emails and Skype calls, and I don't think it hurt me. I got the interview last week to my top choice (and not an easy program to get into) and not once did my GPA come up even though everyone who interviewed me all had access to my transcript. So saying that, I'm not completely sure now if it's necessary to mention ahead of time.
What's important is the whole package, and it's okay if one area is weak if the others are very strong. Having excellent GRE scores for one, but more important will be research experience, even better if you can get your name on a published paper. Letters of recommendation of course, and even better if they're from professors/researchers you've actually worked with. As someone else said, if potential advisors likes you they'll fight for your admission.
Of course, once you're in grad school, make sure you do get good grades!