Personally I wouldn´t buy an Apple notebook right now, but rather wait and see what the Intel switch brings along. The current ´book lineup is somewhat dated, G4 processors instead of G5 etc. That said, I am satisfied with my 12" Powerbook (1.5 Ghz, 1.25G RAM) and current iBooks are pretty good bang for the buck.
In the end you don´t necessarily need a top notch machine for recording, it depends on what kind of projects you are planning to do. Personal projects or bands? Audio or MIDI? External or software synths?
Multi-track recording in itself isn´t very demanding, years ago I could easily record up to 20 tracks (16/44.1) with 350Mhz Pentium II. But when you start to play with real-time plugins and software synths, the system load increases rapidly. However, with careful track economy and sensible use of plugins (using less processor-intensive plugs for secondary tasks, assigning them to groups rather than individual tracks etc.) you can manage quite large projects with surprisingly modest rig.