What Demosthenes X said. After getting an MBP, I can't imagine buying a Windows notebook, but I use a home-assembled desktop as a DAW, and can make a really nice workstation for many $$$$ less than a comparable Mac Pro.
Why doesn't Apple make a reasonably affordable mainstream desktop? The Mac Pro is really, really pricy and neither the iMac nor the Mac Mini offer room for expandability... I don't get the iMac concept at all. Sure, it's aesthetically pleasing to look at, but it's virtually a notebook computer in disguise, with an attached display and no portability.