It's true that processor speeds haven't improved dramatically in the past few years, but you can also blame battery technology. The majority of new phones/PDAs have some variant of the TI OMAP CPU usually running at around 200Mhz. This is plenty fast if you write good code. The only thing stopping the manufacturers from using the 624Mhz (or faster) chips is the battery life and cost. A better CPU could be designed with more cache memory and better power usage, but then that would cost more. The right solution is to use what we've got to the fullest by writing efficient software.
Apple has done a good job with the iPhone; it's just another ARM-based device running at about 400Mhz. 400Mhz is plenty for what it has to do because Apple cares enough to optimize their software.
If you want to see what's possible on a lowly 200Mhz ARM CPU, try some of my software.
L.B.
www.bitbanksoftware.com