SInt32 systemVersion
Gestalt(gestaltSystemVersion, (SInt32*)&systemVersion);

if (systemVersion >= 0x1070){ //OSX 10.7
}else{//OSX 10.6
}

enum {
    MDUndeterminedVersion    = −1,
    MDTiger                  = 0x1040,
    MDLeopard                = 0x1050,
    MDSnowLeopard            = 0x1060,
    MDLion                   = 0x1070,
    MDUnknownKitty           = 0x1080,
    MDUnknownVersion         = 0x1090
};

static SInt32 MDSystemVersion = MDUndeterminedVersion;

@implementation

+ (void)initialize {
    if (MDSystemVersion == MDUndeterminedVersion) {
        SInt32 MDFullSystemVersion = 0;
        Gestalt(gestaltSystemVersion, &MDFullSystemVersion);
        MDSystemVersion = MDFullSystemVersion & 0xfffffff0;
    }
}

- (void)someMethod {
   if (MDSystemVersion >= MDLion) {

   } else {

   }
}

@end