Ran into an issue while needing Python 3.0.1, and OS X's Python install is only 2.5.1 on Leopard.
I didn't want to screw with much, cause there are a lot of problems that could arise by yanking the 2.5 install and replacing it with a largely unsupported Python3.
So I took about 3 seconds and a google search of "update python os x", and came across Python 3.0 On Mac OS X (alongside 2.6, 2.5, etc).
Now, there are OS X Installers, but I like seeing where things go, and its actually kinda fun to compile stuff by hand. So thats what I did.
And that link, while kinda old, is pretty good.