i don't think it will be problem where you're being recommended by professors that you didn't recently take classes with. as long as the letters are written recently (as in dated 2009/2010) then i think you will be fine 🙂
(in my case as an example, i asked my humanities professor whom i took a class during my freshmen year which was 3 yrs ago. but that was the only non-science letter that i could get a hold of. it didn't turn out to be problem when he wrote the letter beginning of this year. i submitted the letter, and the schools accepted it.)
