I would say, more often than not, it is advantageous to be a psych major. It shows your long-standing interest in psychology and that you have a strong (assuming you have good grades) knowledge-base of psychology which you can obviously use in grad school. Also, unlike law school, which accepts students from various disciplines and prefers students to come in as "tabula rosa" or as clean slates, most graduate students in psychology were undergraduate psychology majors. However, it is not disadvantageous, per se, to come from another major as long as you have good Psych GRE scores. However, I remember seeing at least one grad school that wouldn't accept any non-psych majors.