Here is a screenshot of what video ads look like on the YouTube web-app on iPad.
Although the iPhone version of the YouTube website looks like the iPad-optimized one, it still utilizes the regular iOS video player and I haven't encountered an ad yet.
If you really don't want these ads, you can use a third-party YouTube app like Jasmine (made by the amazing Jason of Alien Blue - the reddit client - fame), since the built-in app for iOS is gone with iOS 6.
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