A few reasons why Macs are so expensive:
1. It's a closed hardware system. Apple either manufacturers or has a big part in the design of all of their own core hardware (motherboard, CPU, enclosures, etc.). With this in mind, Apple can charge whatever they want for their hardware. And there are a enough Mac enthusiasts out there to actually buy it. Since the motherboard is an Apple design which isn't available for general consumers to buy, you are generally forced to buy a whole new machine if you want a faster processor.
2. Steve Jobs wants you to buy a fashion statement. Don't get me wrong, I love the Apple platform, BUT.... The fact remains that Apple is technologically about a year behind PC, yet they demand a price premium. It's a niche market. This won't change unless Apple gains a reasonably larger market share as a platform (highly doubtful). A lot of schools used to buy Apple machines EXCLUSIVELY, but now they've figured out that even with the educational discounts, PCs are a lot cheaper and run more software.
3. Apple has a unilateral pricing policy with retail channels. This means that no matter which Apple-authorized dealer you go to, that model you were eyeing at one store will have the exact same price tag at another store. This means no price competition, which keeps prices very high.
I love the Apple platform too, but it'll be a while before I could actually afford the Powerbook G4 I want.