Apple and every other software author that I’m aware of have always used the historic convention of 1024 bytes (B) in a kilobyte (kB), 1024kB in a megabyte (MB), 1024MB in a gigabyte (GB), and 1024GB in terabyte (TB). Hard disk manufacturer’s have always used the convention of 1000000000 B being 1 GB, instead of 1073741824 B being 1 GB.

On the introduction of OS X 10.6, Apple have adopted the convention used by hard disk manufacturers, as explained by Apple at http://support.apple.com/kb/TS2419, updated the day before Snow Leopard became publicly available. This clouds the issue of space being recovered by installation of Snow Leopard, though that is undoubtedly true.

The difference increases to almost 10% when measuring terabytes. Confusion will surely ensue when comparing file sizes between Leopard and Snow Leopard Macs, not to mention Windows and Linux PCs. I sense that this has something to do with the previous legal cases over iPod capacities.