You can read here and there that the new PocketPCs using the new PXA255 chipset have a 200mhz bus (e750) and others (2215)a 100mhz bus... It seems a bit odd and complicated and as usual it's simpler than it looks!
The new PXA 255 chipsets all use an "internal" bus running at 200mhz and an "external" bus running at 100mhz as explained by Intel here. What's the difference between these two "buses"? In fact the "internal" bus is simply the speed of the cache (32/32kb for the most frequently used instructions/data) and the "external" bus, the "real" bus, the one that talks to the outside world, i.e. the RAM, memory cards, display, etc...
To sum it up, the PXA250 has a cache running at 1/4th the speed of the CPU and the PXA255's runs at 1/2, which is nothing to sneeze at, but there's no good reason (other than marketing) for this to be different from one PXA255 based PocketPC to the other!
PS - Here's a little utility (CPUID v1.3) tu check the validity of the above statements ;-)