I think if there were HOWTOs for platforms, e.g. MacOS X, each BSD, each Linux distribution, more people would be mirroring CPAN (e.g. on their laptops).

Like the How to mirror CPAN section of the CPAN FAQ you mean :-)

Basically:

rsync -av --delete rsync.mirror.of.your.choice::CPAN mycpan
[download]

should do the job.