How to tell what version of Mac OS X you have?

The system version is on the Installer disc, at:

/System/Library/CoreServices/SystemVersion.plist

You can get the specific information with this command:

defaults read /path/to/mount/System/Library/CoreServices/SystemVersion ProductBuildVersion

While the above describes how to do it on media, you can also test your local hard drive.  See below:

[nerv:/system/library/CoreServices] benjamin% defaults read /System/Library/CoreServices/SystemVersion
{
ProductBuildVersion = 10F569;
ProductCopyright = "1983-2010 Apple Inc.";
ProductName = "Mac OS X";
ProductUserVisibleVersion = "10.6.4";
ProductVersion = "10.6.4";
}