In one scene at a birthday party, I believe, a girl offers to show the guy her breasts, because she made a promise to do this if he ever graduated, which he did.

I've seen that but can't remember which movie it was either. I've tried searching.
I didn't see it too long ago and so I'm going to guess at this one. I don't have the film so can't check myself. Maybe?

Nick and Norah's Infinite Playlist (2008)

Thanks for your response. That was not the movie that I was thinking of, but your answer sparked my memory with "Cougar" and so the search of my databse found it. Cougar Club with Nicole Rayburn.

I am about to post the clip in the newsgroup alt.binaries.multimedia.nude.celebrities if you are interested.

Very silly and funny clip.