There's a special TEDx going on in Geneva today, devoted to Transmedia. The first thing to understand is those who like Transmedia thinks its different from Crossmedia. Frankly I have always interpreted Crossmedia to be this. That's where an immersive story is told through across various platforms, which could be web, phone, TV, or radio. Anything else is merely cross-platform story-telling. But OK, we're stuck with the term.
I do love the You Suck at Transmedia website. Because at the moment, there are not many good examples. And the problem is that good transmedia productions don't lend themselves very well to repeats. The Truth About Marika is a good example. So they end up being relatively expensive. But the Marika project was brilliant at the time. I am surprised that the company that made it doesn't understand how to get the aspect ratio right on Youtube.