He said as much in an interview he did in London while promoting My World 2.0. Basically he said that it depends on the situation, but that he would do it. Justin isn't only up for dating other stars, he would date anyone if she was the right sort of girl.
And another detail: Justin likes girls with nice smiles, pretty eyes and who can make him laugh. This is his description of his dream girl. Do you fit the mold? The you may be just right for Justin!