Snow Leopard: What you should know about… 64-bit
Most Macs won’t boot into the 64-bit kernel by default. Many Macs can’t boot into the 64-bit kernel at all, even though they have a 64-bit CPU and firmware.
For the most part, this doesn’t matter. Unlike Windows (and like Leopard), Mac OS X can run both 32-bit and 64-bit applications under a 32-bit kernel, provided the hardware supports them.
Snow Leopard is a lot more 64-bit-savvy, though. Nearly all of the applications included with the OS will run in 64-bit mode by default if your hardware is capable of it.
The practical outcome of running with a 32-bit kernel is that your kernel extensions don’t break. If you have, say, a 3G dongle, or perhaps run VMware, this is a Good Thing.
It wouldn’t be surprising if Apple adds support (along with driver updates) for booting into the 64-bit kernel by default on more Mac models as part of point release updates.
Apple has published a technote on the subject, and there’s an additional note as well as a troubleshooting document for Mac OS X 10.6 Server.
