In the meantime he's written an episode of season two of Game of Thrones, Blackwater, which sees a massive military engagement. He is also working on a coffee table book called The World of Ice and Fire, which will be maps and genealogies and history and stuff.
The interview gets into some interesting spoiler territory, especially about the thematic meanings of some events in the last two books. Most interesting to me, though, are Martin's thoughts on how to write magic:
I can tell you generally that when treating with magic in fantasy, you have to keep it magical. Many fantasy writers work out these detailed systems, and rules, and I think that's a mistake.Ice & Fire
For magic to be effective in a literary sense, it has to be unknowable and strange and dangerous, with forces that can't be predicted or controlled. That makes it, I think, much more interesting and evocative. It functions as a symbol or metaphor of all the forces in the universe we don't understand and maybe never will.