Update: These instructions are over a year old, though they may still work for you. See the “Install Python” page for the most recent instructions.
A bit ago a friend and I both had fresh Mac OS X Lion installs so I helped him set up his computers with a scientific Python setup and did mine at the same time.
These instructions are for Lion but should work on Snow Leopard or Mountain Lion without much trouble. On Snow Leopard you won’t install Xcode via the App Store, you’ll have to download it from Apple.
After I’d helped my friend I found this blog post describing a procedure pretty much the same as below.
Update: If doing all the stuff below doesn’t seem like your cup of tea, it’s also possible to install Python, NumPy, SciPy, and matplotlib using double-click binary installers (resulting in a much less flexible installation), see this post to learn how.