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\";
}