Using a proportional font will break if you try to align code other than using indentation, e.g. aligning assignments.

More examples: http://mysite.verizon.net/astronaut/vim/align.html#Examples

Of course, I really hope nobody's coding like that. I've never seen anyone who's been able to maintain that kind of formatting consistently.

I don't see any reason why proportional fonts shouldn't work for most programming styles. The problem is that most existing proportional fonts aren't very well suited for programming. At the very least, you need a font with unambiguous shapes for similar glyphs, like 1lI0oO`',.;:

Leading spaces aren't much of a problem, but aligning comments on the right-side is much harder due to the variable widths of each line. And even if you align them in your editor, they'll be misaligned for people who use fixed-width fonts. Does that make any sense?