Buy now!
To me the changes to the 3G S are significant enough to justify not waiting: better camera w/ video, video editing capabilities, compass, 32G capacity, voice control, 2X faster, better battery life.
Also, think of all the cool apps that will be coming out in the next few months to take advantage of all the new features.
I view it like trading in a car. I buy what I want now and if something comes out later, I can always trade-in what I have (i.e. sell current iPhone on eBay) to off-set the cost of the next purchase. Only down side is that you have to deal with at&t on your contract issues.