How do I launch a YouTube video in my iPhone app?

The iOS device has URL schemes that it recognize. Build your youTube url like the following:

http://www.youtube.com/watch?v=VIDEO_IDENTIFIER

That will launch the youTube player on the device.

NSString *videoName = @"1JynBEX_kg8";
NSString *string = [NSString stringWithFormat:@"http://www.youtube.com/watch?v=%@", videoName];
NSURL *url = [NSURL URLWithString:string];
UIApplication *app = [UIApplication sharedApplication];
[app openURL:url];

For more information on iOS URL Scheme Apple URL Scheme Reference.