The iPhone 3GS runs on an 833 MHz Samsung chip, that, presumably to increase battery life, is underclocked to 600 MHz. While this is better than the original iPhone and iPhone 3G's 620 MHz CPU (underclocked to about 412 Mhz), there's still plenty of room for improvement. Analysts fully expect that improvement will come either from the A4 chip itself or a lower-powered variant of it designed for the iPhone's smaller screen and battery.
Early impressions of the iPad's speed from people who have actually handled one are that the device is far faster than any iPhone or iPod touch released so far, with applications opening "instantly," and provides far smoother graphics performance. With Apple now designing and implementing its own "system on a chip" CPU for the iPad, it seems very likely this will be one of many iPad features that will trickle down to Apple's smaller mobile devices. Once the iPad actually finds its way to consumers (and teardown sites), we'll have a much better idea of what Apple's A4 chip is capable of. As for the next-gen iPhone's CPU, I'm placing my bets on an A4 variant rather than the full iPad CPU, with an operational speed in the neighborhood of 800 - 850 MHz -- more than twice the speed of the iPhone 3G.