After a recent internal software reshuffling over at YouTube's servers, however, those third-party clients started misbehaving and lost their ability to play videos in high quality. Fortunately, those problems have been fixed now after updates to both Supertube and LazyTube took care of the issue.
As of now, it is only YouTube, as the free version of LazyTube is known in the Windows Marketplace, that has not been updated yet, but its developer has made it clear that a fix resolving the problem should be out very soon. Grab the updated versions of the apps from the links below, and if you have not checked them out so far, this may be the perfect moment for you to do so.
Supertube [Zune link]
LazyTube [Zune link]