This isn't an issue with just Apple laptops. Most every laptop out there is only upgradeable to a certain extent. You can upgrade the RAM, HD, and that's about it. Everything else is basically a proprietary design since it has to be made small and light enough to be portable. This isn't so much an issue with desktops since they're meant to be in one place. Whether you get an Apple, Dell, Gateway, etc., your'e only going to be able to upgrade it so much.
Now, there are some laptops that allow you to replace the CPU and some other things but these laptops are meant to be desktop replacements or portable desktop systems. These laptops basically use desktop CPUs and are usually very heavy, have poor battery life, and generate a LOT of heat. Now you might say that the Pentium-M would be able to make up for this and I do believe there are Pentium-M upgradeable laptops, but again you're talking about something that wouldn't be designed with portability in mind and that's assuming that it's using the same components that make up the Centrino (IntelPRO wifi, Intel 9xx chipset, Intel Pentium-M/Core Duo or Solo).
Again, it's not specifically Apple but anyone that's manufacturing a laptop nowadays.