I checked my H2/sling yesterday after reading this thread and mine is missing also. I have a voice command remote so I can get it that way but if I had the older remotes I would have to go to the guide ch.371 to get it because it's not in the apps at all.
I still have the YouTube app on my H3 as well. But it won't let me update to 536. I suffered numerous anomalies last night, though. Everything (live channels, recorded programs) were unplayable because of horrendous stuttering. Rebooted the Joey with no change. Rebooted the Hopper and that fixed it.