A video entry's media:content elements contain the URLs for the playable video. The yt:format attribute indicates the kind of video for each media:content.
For example, here's a snippet to print out the URLs for all videos in Flash format (yt:format=5) in a feed:
for (GDataEntryYouTubeVideo *videoEntry in [videoFeed entries]) {
GDataYouTubeMediaGroup *mediaGroup = [videoEntry mediaGroup];
NSArray *mediaContents = [mediaGroup mediaContents];
NSNumber *flashFormatNum = [NSNumber numberWithInt:5];
GDataMediaContent *flashContent;
flashContent = [GDataUtilities firstObjectFromArray:mediaContents
withValue:flashFormatNum
forKeyPath:@"youTubeFormatNumber"];
if (flashContent != nil) {
NSLog(@"%@", [flashContent URLString]);
}
}
Also take a look at this blog post on playing videos on the iPhone:
http://apiblog.youtube.com/2009/02/youtube-apis-iphone-cool-mobile-apps.html