[chucker]

Originally Posted by bassplayinMacFiend Not to mention that at this point in time, 64bit GUI programs aren't supported in OS X. Only 64bit command line programs are supported.

That doesn't make much of a difference, however. It poses two limitations:

• to take advantage of 64-bit, developers are forced to split front-end (GUI; 32-bit) and back-end (CLI; 32-bit/64-bit). They should, however, be doing that anyway, as it greatly increases efficiency no matter what.
• here's the kicker: if we're talking applications that need to display something in the GUI with 64-bit precision -- an extremely detailed 3D model, for instance, then you're out of luck with OS X: you'll have to convert back to 32-bit in the process. That means you either have to interpolate, thus lose precision, or recalculate, thus lose performance.