First and most important thing to note, Centrino is NOT a processor, don't get it mixed up. Centrino is a technology based on three important features, that is the intel Pentium M processor, integrated wireless networking, and intel 855 Chipset.
The main aim of a Centrino laptop is to achieve true ease of mobility. Lower clockspeed translates to less heat and less energy consumption, which means longer battery life, quieter, cooler, smaller, and lighter laptop. Integrated wireless brings convenience to wireless networking whereever you are, whenever you need it. Not sure what the 855 chipset does, but it is also a very important feature of a Centrino. Without any of these 3, the laptop won't be a Centrino.
To make up for lower clock speed, Intel Pentium M Processor has larger L2 cache, that is 1mb of it. Newer 'Dothan' Pentium M processor has 2mb of L2 cache. Don't underestimate this L2 cache, because its what makes a 1.4Ghz Centrino performs on par with P4 2.8.
All these prove that clock speed isn't everything, so when getting a desktop, get Athlon instead!

Btw, why is this in bar?