システムバージョンの取得
方法①
SInt32 version = 0;
Gestalt( gestaltSystemVersion, &version );

if ( version >= 0x1050 )
{
// New Leopard specific drawing code.
}
else
{
// Old drawing code.
}
//gestaltSystemVersionはOS >= 10.4.10 doesn't work.

//gestaltSystemVersionMajor
//gestaltSystemVersionMinor
//gestaltSystemVersionBugFix
/System/Library/CoreServices/SystemVersion.plist

別方法：
{
 NSString* osVer = [[NSProcessInfo processInfo] operatingSystemVersionString];
 NSScanner* scan = [NSScanner scannerWithString:osVer];
 
 NSString* sVer;
 [scan scanUpToString:@" " intoString:nil];
 [scan scanUpToString:@" " intoString:&sVer];
}

方法②
NSAppKitVersionNumberでOSバージョンも取得できそう、実装はまだわからない
方法③
ターミナールで/usr/bin/sw_vers　コマンドでversionを表示できる
方法④
A better way to obtain version information in Mac OS X is to read the system version information from the following file:
/System/Library/CoreServices/SystemVersion.plist