I'm in the same boat as jenner, coming from Biochemistry and (hopefully) going into tissue engineering. I've asked professors and admissions reps about how willing BE/BME programs are to admit biology students and all of the ones I've worked with have been more than happy to extend offers to biologists (I've worked in tissue engineering, drug delivery/nanomaterials, and stem cell biomechanics), provided that they have demonstrated some math, physics, and programming aptitude. They say that certain professors are prejudiced against biology (particularly in biomechanics), but depending upon your discipline you should be able to find a good fit somewhere. Also, I've been told that if the BME program comes from the medical school (like Johns Hopkins) they are more likely to admit biology students than if it grows out of the engineering school (MIT, UCSD).