A lot will depend on your state's certification procedures.
NCLB "highly qualified" teacher mandates will have an impact also.
What kind of certification will you have?
If you have a general science certification, then you might have to teach physics or chemistry; and it would be much better for every one concerned if you are comfotable with the content.
Something else to consider - in Texas there are a lot more biology majors than there are biology teaching spots. Some biology majors get other courses like environmental science or aquatic science, but many, many of them end up teaching middle school. This is what happened to me. It was good for me though, because I found my niche, but if I had been set on HS bio...
Another thing to consider (again in Texas) lots of PE majors & wanna be coaches get biology certification because they take so many bio course anyway (general bio, anatomy, physiology...). I
When a HS has a choice between a straight bio teacher and a bio teacher/coach, oftentimes the coach will have the upper hand.
I would advise you to make yourself as marketable as you can.