Are certain videos blocked by Youtube on the iPhone or is this a technical issue? When I search for any major label band on my iPhone 4 almost none of the official video clips are available.

I can always find the equivalent clip on DailyMotion.

I used to be able to find more results on the "classic" version of Youtube, but Youtube now always redirects to the mobile version. This started sometime in the past couple of months.

Say for example any of MGMTs official or VEVO videos or Pumped Up Kicks by Foster the People. These can all be found and watched on a desktop browser, but can't be found anywhere on my iPhone.

Is there some licensing issue?
There is a syndication setting that you choose when you upload a video to youtube in the video settings page under Broadcasting and Sharing Options that allows it to be played on mobile devices or not.