Angie, I faced a prof a year ago at a conference that I had let down (big time) during my undergrad. He barely remembered. We had a pint together, and then I mentioned it and we had a good laugh. In your case, if the prof is a decent person at all, they'll be happy to see that you're on your feet and running. I'd say say hi, have small talk,and don't bring it up unless he does. Don't avoid him. Just tell him what you're working on. Try to move on. Good luck!