Despite numerous bug reports and detailed explanations to the relevant people at Apple of exactly why it sucks, Core Graphics reports most (all) LCDs as having a refresh rate of 0 fps (and doesn’t allow vertical retrace synching except that built into OpenGL). The standard work-around is to treat 0 as 60; the “lag” of current LCD technology means that faster refresh rates are pretty pointless anyway.
You can see if this is the case in System Preferences – if the Refresh Rate pop-up menu is greyed out and reads n/a, the refresh rate is being reported as 0. (The interface may be different on pre-Tiger systems.)
On systems other than Mac OS X, the cap will be whatever SDL says the refresh rate is.