When an OS is distributed with some specific Python release and uses it for some OS functionality (as is the case with Mac OS X, as well as many Linux distros &c), you should not tamper in any way with the system-supplied Python (as in, "upgrading" it and the like): while Python strives for backwards compatibility within any major release (such as 2.* or 3.*, this can never be 100% guaranted; your OS supplied tested all functionality thoroughly with the specific Python version they distribute; if you manage to alter that version, "on your head be it" -- neither your OS supplier nor the PSF accepts any responsibility for whatever damage that might perhaps do to your system.
Rather, as other answers already suggested, install any other release you wish "besides" the system one -- why tamper with that crucial one, and risk breaking things, when installing others is so easy anyway?! On typical Mac OS X 10.5 machines (haven't upgraded any of my several macs to 10.6 yet), I have the Apple-supplied 2.5, a 2.4 on the side to support some old projects not worth the bother to upgrate, the latest 2.6 for new stuff, 3.1 as well to get the very newest -- they all live together in peace and quiet, I just type the release number explicitly, i.e. using python2.6 at the prompt, when I want a specific release. What release gets used when at the shell prompt you just say python is up to you (I personally prefer that to mean "the system-supplied Python", but it's a matter of taste: by setting paths, or shell aliases, &c, you can make it mean whatever you wish).
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