The default filesystem is case-insensitive. Yeah, unlike all boxes in the Unix world.
It's actually slightly worse than that... I don't recall the circumstances in which it's bitten me, but OSX's default filesystem is sometimes case-sensitive and sometimes (most of the time) not. I have no idea what the guys at Apple were thinking when they allowed that to happen.
That single issue aside, though, I'm currently on my second PowerBook and have found it to be a wonderful platform, both for Perl development and for doing unixy things in general.