I use the custom url schema defined as follows: youtube://user/[username]. I didn't find documentation for this but discovered it by means of experimentation. Here is a also a cool way to test if the native App is installed.
NSString* social = @"http://www.youtube.com/user/CanadianMortgageApp";
NSString* socialApp = @"youtube://user/CanadianMortgageApp";
if ([[UIApplication sharedApplication] canOpenURL:[NSURL URLWithString:socialApp]]) {
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:socialApp]];
} else {
[[UIApplication sharedApplication] openURL:[NSURL URLWithString:social]];
}