The ARM CortexTM-A8 processor is the first applications processor based on the ARMv7 architecture and is the highest performance, most power-efficient processor available from ARM. With the ability to scale in speed from 600MHz to greater than 1GHz, the Cortex-A8 processor can meet the requirements for power-optimized mobile devices needing operation in less than 300mW; and performance-optimized consumer applications requiring 2000 Dhrystone MIPS.
Folks, the N900 has the same exact chipset and scales to 600mhz with no problems (I own one and the device goes to 600mhz playing Duke Nukem, MAME and Quake). The chipset can run at 1ghz (but unstable). The talk about Droid being unstable at 600mhz must be due to other issues and not the actual scaling to 600mhz.
The processor in the DROID may max out at 650 MHz, but 650 MHz will be its max.
The OMAP3430 (the Droid's) runs stock at 600mhz, with the droid @ stock underclocked to 550mhz. The OMAP3440 runs stock at 800MHz, while newer models, such as OMAP4 will have speeds upwards of 1GHz and higher. It's not recommended to clock the Droid past 600mhz, even though it's not possible for now.
Retired: Motorola Droid, HTC Droid Incredible. VZW iPhone
Current phone: Droid Bionic