A lot of *nix apps will work on mac os x either in a Cocoa native
version or using X11. There are various package management systems to
help you with this if you like, such as Fink, Darwinports and Gentoo.
They all have their pros and cons, for instance I've found that Fink's
packages are often a little out of date, while the other two usually
build from source, which can be an inconvenience.
As far as audio applications go, if the application can use portaudio,
jack or coreaudio, you're probably fine, but if it needs alsa then
you're not. This applies to midi as well, which is quite a limitation at
the moment, but will become less so as jack midi catches on.
Apart from this, I can't think of any ways in which you are likely to be
worse off running intel mac os as opposed to Linux, except that it is
probably possible to squeeze extra performance out of Linux system. From
what I understand, the Mach-based kernel that OS X was quite
inefficient, although I have also read somewhere either that this has
changed somewhat or that it is going to change sooner or later. My
memory is a bit foggy here, so you'll have to do your own research.
>
If you want to run an OS apart from Linux, you're much better off with
Mac OS than Windows. There are, in any case, a number of ways to run
Windows apps in Mac OS.
There are a number of websites addressing Linux laptop compatibility.
The Ubuntu wiki I think has a good section on this. This can be a very
tricky area. Chances are that intel macs will be relatively well
supported like their predecessors, but you will need to do your
research. One advantage of a mac is that if you do have problems with
Linux hardware support, then you will at least have the option of
switching to another quality OS that definitely "just works".