My pre-req and intro course grades were downright attrocious. This was brought up during my two interviews. My grades for upper level courses, were 1 B+, 4 A's, and 4 A+'s. It was brought up during one interview, where the dude basically asked me if I was lazy or a diligent idiot. Be prepared to answer about your studying habits, what's different with your approach now, and why/how you can expect to maintain a high level of quality grades.
It also helps to have a decent DAT as well, to backup your claim that those C's/D's/F's were an aberration.
But to answer your question - I'm definitely concerned, because I haven't received an acceptance anywhere yet. Worse comes to worse, my personal letter for my re-application is almost finished, my additional rec letters are soon to written, and I'll be ready to go again in May.