If your major isn't math or a science, you'll have fewer science classes, so your BCPM GPA will be based on a relatively small number of classes. Thus, the important thing is to do well in every math and science class you take - a single slip-up can hurt you badly. As long as you handle your science classes, though, your non-science major can only work to your advantage. Because there are so many similarly qualified people applying, the difference between accepted and rejected applicants often comes down to the presence or absence of some aspect(s) of their application that stands out as unique. A non-science major will definitely help set you apart from the crowd. Plus, keep in mind that if you major in a subject you truly enjoy, maintaining a high GPA won't be such a chore.
There was a thread a little while back that had a link to average MCAT performance by major. To summarize, humanities majors scored highest on the verbal, but math majors(!) scored highest on the sciences and overall. Of course, these statistics don't really matter; what's important is your own preparation for the MCAT.