NSUserDefaults on iPhone SDK not returning nil

NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
if ( [defaults valueForKey:kIdentifierKey] == nil ) {
    return YES;
}
else {
    NSLog(@"we have a value. return NO. Value is: %@", [defaults objectForKey:kIdentifierKey]);
    return NO;
}

if ([defaults boolForKey:kIdentifierKey]) [self doSomething];

[defaults valueForKey:kIdentifierKey];
[defaults objectForKey:kGroupIdentifierKey];

if (![[NSUserDefaults standardUserDefaults] valueForKey:@"foobar"])         NSLog(@"is not yet defined");
if (![[NSUserDefaults standardUserDefaults] objectForKey:@"foobar"])        NSLog(@"is not yet defined");
if ([[NSUserDefaults standardUserDefaults] valueForKey:@"foobar"] == nil)   NSLog(@"valueForKey is nil");
if ([[NSUserDefaults standardUserDefaults] objectForKey:@"foobar"] == nil)  NSLog(@"objectForKey is nil");

is not yet defined
is not yet defined
valueForKey is nil
objectForKey is nil