I'm not defending the MAG250 nor saying it's the greatest thing, but it is hardly a PoS. I have development experience with the MAG and I can say that it is an OK box on the cheaper side. It's widely in use by many broadcasters (although not in America, as in the continent) and has been in the market for many years.
The video you saw shows the internal "portal" which is a little outdated and ugly I admit, but when developing an app you can make the GUI look however you want, even identical to Roku's. A point was made that perhaps the Roku didn't have enough hardware power to do WebKit, and since you asked, I mentioned the MAG which does have less hardware power.
Now, if you compare it to the Roku, yes, the Roku is better: it uses top-notch hardware, the GUI looks beautiful out of the box, it's simple, really pretty and well designed. That's one of the reasons I think it would be amazing to have WebKit capabilities on it.
I'm not trying to be an ass, but I cannot point to a specific app or example of a WebKit GUI in action, in fact I'm not aware of any publicly-licensed framework for building WebKit STB GUI's out there, you could develop your own one though. I can tell you that a lot of industry-standard middleware and suites used by broadcast-quality solutions are WebKit based. Also, the app frameworks for some smart TV's such as Samsung's and LG's are WebKit based as well.
This is not a wild and new idea guys, you might have already seen a WebKit GUI in an embedded device or player and not noticed it.