The interesting thing is, which didn't apply to the first iPhone, is who would buy an obsolete product, KNOWING FOR SURE, that a faster version is going to be available that imminently?
Well, "imminently" is relative. I think someone (you?) said that the FCC can only delay the release of the report by a month or so. Apple would only have to endure, oh, maybe 1.5-2 months of decreased iPhone sales -- not great, but not a horrible situation, either. I'm (wildly) guessing that Apple would need around 3 months of time to produce sufficient quantities of the new iPhone, after getting FCC approval (~1 month of which would be "hidden" from the public via the delaying of the FCC report release).
I don't think credits would be offered. After all, people would likely have 1-2 months of warning. If they're crazy enough to buy the old iPhone when they know a new one is around the corner, they really shouldn't expect any credits. However, Apple being Apple, you might see credit for people who bought the old iPhone within, oh, the last 2-4 weeks or so, but I really wouldn't expect that.
Besides, I think 3G is overrated (and coverage is still spotty, IMHO) -- while I'd love to have 3G, even wifi speeds are too slow for me. The iPhone really needs a faster processor (and, who knows, maybe we'll get that, too?). When the new iPhone is announced, the real deciding factors for me are memory capacity and the existence of a real GPS. Give me 32GB or more, and a real GPS, and I'm sold (but I'm wondering if AT&T will raise the rates for 3G access, or something else).