Psychology is considered a "soft science". If your school's admission committee is composed of people with degrees in biology and chemistry, they will laugh if you send your psychology as a science LOR. Look at the website and if you have a specific question, just e-mail someone in the admissions office. It's better to contact them instead of base your decision off of something on here. I am submitting a psy prof's LOR, but my school does not have a requirement of who the letters come from. They just require 3, one being a dentist.
Pysch wont really count as a science professor, some schools may accept it but I wouldnt rely on it. If you were really really close to your psych professor and he/she will write you a great letter, I would include it in your letters but dont rely on it as a science, some schools will be very strict about this.
my letters were from a Nutrition prof, Anatomy prof, Head research prof and my dentist... I interviewed at 3 schools before Dec 1st (and got in to all 3) and i was invited to other 4 schools for interviews after Dec 1st (i didnt go to those) ... my DAT's were blah! and my GPA was good! my point is that as long as the letters are from ppl that know you well and will write a good letter you are fine! dont sweat it!