They could finally be getting around to making <link=http://half-life.wikia.com/wiki/Prospero>Prospero, which was one of the game ideas that they were kicking around before the original Half Life was released. The article I linked said that the game was partially inspired by the art of Jorge Luis Borges, and a quick google image search turns up a lot of caricatures with messed up proportions, similar to what is shown in the concept art. Gabe Newell said that they might start working on it after finishing TF2, and this may be a sign that they've been doing just that.
Edit: Apparently, Borges was an author, not an artist; however, the stuff he wrote was pretty darned weird, and may still be an influence on the concept art we're seeing.
Edit Edit: Looks like I was ninja'd twice over on the idea. Nice to see that it's not just me being crazy...