an appropriate use of sarcasm IMO. Agreed, the boundaries between SUL and fastpacking are somewhat arbitrary. I think with 'fastpacking' being a relatively new term the definition is still getting hashed out on forums like this. I do think there is definitely a distinction though, just not a clear one. IMO, fastpacking has more to do with your mode of travel than how much your pack weighs, though obviously less weight makes it easier to travel fast.
I think fastpacking is a good word to use for fast/light high mileage trips, where there is often running involved. If I want to cover a long distance like 60 miles and I only have 24 hours to do it, then I'll probably just throw a bunch of cliff bars and peanutbutter, and the bare minimum gear for an overnighter in a backpack that rides well when I'm running in it. I'll hike fast on the ups and run the downs. It doesn't really matter to me what I call that, but its definitely a different approach then packing light for a nice walk in the mountains. Both approaches have there benifits, but the first is much more about the athletic challenge than the second.
For gear, I see fastpacking used most often for backpacks. I think this is related to how the pack rides when you're running. Its also often used for super minimalist equipment, which will help you survive the night but not necessarily in a lot of comfort.
Not trying to be authoritative or put out a definition, but I don't think its the definition is completely arbitrary either. Then again, in most situations it hardly matters what word you use. If my friend asks me what I'm up to this weekend, I'll probably just say 'backpacking,' regardless of how much my pack weighs or how fast/far I plan to go.