How can I tell if my machine is 32 or 64 bit?

To determine if you computer is running 32-bit or 64-bit on Windows, follow these steps available from Microsoft's KB Article: 827218

Mac OS X 10.5 (or greater) is a 64-bit operating system. Intel Core 2 Duo, Intel Xeon, and PowerPC G5 are 64-bit processors. All currently shipping Macs have a 64-bit processor and a 64-bit operating system.

Linux users should type the "uname -a" command.