As for buying, go for a dothan processor (newer, similar technology) if you can afford it - but this is certainly the way to go rather than down the line of huge desktop replacements that are too heavy to carry around.
The Centrino label is not important. The Pentium M is.
To follow on from what Diodorus Siculus was saying, from what I can remember, for a laptop to be called Centrino, the 2nd requirement is Intel's own wifi connection (IntelPro Wireless), the 3rd is Intel's motherboard chipset (something like 855 something or other).
The most important item is the Pentium M CPU. It's the new architecture of the Pentium M which gives it better performance than the Pentium 4 despite a lower clock speed, and more efficient power consumption compared to the old laptop Pentium CPUs.
The chipset almost inevitably comes with the CPU (I'm not sure there are any makers who use the Pentium M without using the chipset).
As to wifi, some laptop makers use another wifi adaptor (sometimes better than the Intel one). As a result, these cannot be called Centrino.
If a laptop says it uses Pentium M, that's all you need. Don't worry about the Centrino label.
The Dothan is a more recent core for the Pentium M. 2MB L2 cache (double that of the last Pentium M core) - except perhaps an ultra-low voltage one, and further improvements on power management. New laptops should be offering the Dothan core Pentium M.