Hey,
Laptop G4 does not equal desktop G4. While the chips may be similar or the same, there are other things to take into cosideration, like bus speed, drive speed, ram speed, heat, etc.
Laptops are designed to be portable and their speed is secondary; desktops are designed to be fast, and their size is secondary.
A HUGE disadvantage of a laptop is not being able to use 99% of the soundcards available, you're stuck with the USB and firewire cards which, in my experience, haven't been as reliable as their PCI bretheren.
Sure, you can use the onboard card, but you'll soon cringe as you discover the world of noise and latency!
There is also expandibility: I am typing this on my Dell inspiron 8100 laptop which I've had for 2 years. When I got it, it had the FASTEST laptop processor at the time (1.2ghz PIII), 256MB RAM, 30GB 5400 RPM hard drive, cd-r, DVD rom, etc. It cost a cool $2000.
Nowadays, you can get latops which destroy mine in speed and size for a fraction of the cost, and the only thing I can do to make mine any better is to buy a bigger hard drive, max out the RAM to 512MB, and maybe get a new battery.
It's true that in the computer world things become obsolete very quickly, with the laptop they become VERY obsolete even more quickly, and there's not a lot you can do to prevent it.
Dollar for dollar, the only advantage you have with a laptop is that it is easy to move around with you.
Take care,
Nick