I don't know much about processor speed, but I'm pretty much decided on the $100 question. I'm going to wait as long as I can because I figure either:
a) the update will boost standard memory, possibly saving me > $100 in upgrades;
or:
b) today's ibooks will be discounted $100 or more after the new ones come out.
It's possible that the update will bring nothing that I really care about (say bluetooth and a minor speed bump), but if that happens, at least when I buy my "outdated" model, I'll be making an informed decision.