All Centrino laptops feature a Pentium M processor, Intel 855 chipset and Intel PRO/Wireless 2100 (wireless capabilities). If not all three are included, then the laptop can not be branded as a Centrino but I haven't seen any companies yet that have the Pentium M processor without the other two.
Now about the clock speeds. Pentium M is a whole new processor, that has nothing to do with Pentium 4. Its design is very different, and as a result it has much higher IPC, which means that clocked in lower speeds it can do the same job as a Pentium 4. If you search around for a few reviews, you will see that a Pentium M processor has the same speed as a 50% higher clocked Pentium 4 (e.g. 1.6GHz Pentium M = 2.4GHz Pentium 4).
The advantages of Pentium M processors, when compared to Pentium 4, is that they run much cooler, so you get very thin and light notebooks, which also produce less noise (no significant cooling necessary) and less heat.
I hope this helps.