Intel let it slip that the iPhone uses a Marvell Xscale chip. Intel would know too, since they used to own this line of chips and sold it to Marvell just last year.
It is likely a Marvell PXA900 series ARM architecture CPU, which is described as "an advanced total system solution for today's GSM, GPRS, and WCDMA mobile phones". The performance of this chip is supposedly in the 1000 MIPS range, which would make it about 20% faster than IBM's 849 MIPS 366 MHz PowerPC G3 750CX used in the second edition Firewire toilet seat iBook.
Too bad OS X 10.5 Leopard doesn't run as fast on old school G3 Macs as the stripped down version of it reportedly does on the iPhone.