CNET:
Apple’s Snow Leopard operating system, released Friday, by default loads with a 32-bit kernel, despite running 64-bit applications.
While Mac OS X version 10.6 ships with a number of 64-bit native applications, the kernel itself defaults to 32-bit, unless the user holds down the “6” and “4” keys during boot time, at which point the 64-bit kernel is loaded. Only Apple’s X-Serve products, using Snow Leopard Server, boot into a 64-bit kernel by default.