Agree with gfen. The difference is where the weight is. If you are casting a line, with a lure that happens to be on the end, it's fly fishing. If you are casting a lure, which happens to be attached to you via a line, it is not fly fishing.
Note, that this would allow the use of bait while fly fishing, or the use of flies while spin fishing.
The key here is that to me, there is nothing whatsoever wrong with non-fly fishing. One's not better than the other. There's nothing superior about a woolly bugger when compared to a spinner. The debate is purely academic, a way to define 2 fundamentally different methods of accomplishing the task of catching a fish, but have some gray area in between. It's not about determining which is "superior".
For instance, using lots of lead to dredge nymphs after a "lob" cast is not inherently fly fishing. If you take the next step, and lose the fly line, and go with an all mono leader, it makes no difference. It's not fly fishing. And there ain't a dang thing wrong with it, either. I will do so happily and not think twice about it.
Fly fishing vs. pinning vs. spin fishing vs. trolling vs.....
As a separate academic debate from:
Artificials vs. bait.