Uhh, actually rule #1 is NEVER buy right after refresh/launch. You never know what problems will crop up in that first batch. But yes, I would suggest only waiting two months, buying the iPhone 4 right now seems somewhat unwise. On the other hand, since you use your phone multiple times every day, I figure this is one area where it makes a lot of sense to spend your money. There really aren't many things that use as much as your phone. If you can make that experience better, then it is probably money well spent.
Also, iPhone 5 is probably not going to be a major upgrade. The screen, for example, basically can't get better. The size, for example, can't get appreciably thinner without being increasingly harder to hold. So is it worth it to keep using your beat up Blackberry Curve for another five months so that you can get the iPhone 5 (maybe, if it comes out in CDMA form at the same time and maybe if there isn't a shortage and a backlog for a month)?