It really does come down to how much a person plans to store on their iPhone. In my case, I intended to get the 8gb if available, but knew I'd "settle" for the 4 if that's what was available. And that's what happened-they were out of 8 on "iDay" (I had gone to the AT&T store) but still had 4's so I went home with a 4, knowing I could exchange it if I wanted to. And indeed, when I learned that the Apple store had plenty in stock afterall, I could have easily returned my 4 to the AT&T store, swallowed the re-stocking fee, and purchased an 8 instead. I chose not to bother. Not worth the effort in my case. But for an 8 to have been available in the first place, for $100 difference, sure, why not! Now, if there was a much bigger price difference between the two, I think there would be a lot less 4gig bashing. (LOL reminds me of the Dr Seuss "Sneeches" story)
Afterall, technology wise, they're the same darn thing. One just has more storage, which for many people doesn't matter. In my case, I have about 11 whole whopping songs on there, and a handful of photos, so I have way over 3gb still left. So for me, like I said-not worth the bother.