I am currently designing/building a programming editing environment around proportional fonts (specifically, some kind of sans serif). You can see the effect in this essay I submitted sometime back on hackernews:

http://research.microsoft.com/en-us/people/smcdirm/liveprogr...

Its been fun getting font metrics working right (given a custom editor) and getting key-up and key down to work correctly, but overall, I think this is the future, especially in richer editing environments.