Way cool! Love Jens. Super nice guy, and now that he has discovered Twitter @thejense he has made himself accessible to so many more.
To answer the question: "who is he?" Jens Voigt is a German professional road bike rider. While he is a top rider in his own right, he is what is known as a super domestique. He is universally loved by teammates and opponents. Very gentlemanly, and very funny. He's tough and when he puts the hammer down (which is often) he brings a world of suffering to the rest of the peloton. I'm not going to look it up, but I'm sure his wiki page has a ton of info on him.