Molyneux's next big project on GameSpot has word that "when not contributing to the design of Black & White 2, Peter Molyneux works on a new project code-named Dimitri." The article is based on visiting with Peter in Lionhead's UK-based offices. While there were not many details to be had, there were hints that the game will be for the PC, though there's no way of telling if the "separate discussion" they describe having with him about RPGs being his favorite genre relates to this project as well. Here's a portion of the article that talks of what they did learn about this "Dimitri" project:

In Lionhead's central office, Molyneux is working on a completely new game concept that's code-named Dimitri, a name that will certainly change before the official announcement. He couldn't give more than a few hints about what the project involves, but he said it's very different and won't be a god game like Black & White. However, some of the technology being developed for Dimitri will be worked into Black & White 2. There was some hint that the game will be for the PC, and Molyneux said that game concepts for PCs and consoles will necessarily work differently because mouse interfaces don't translate well to game controllers and vice versa. He said he's exploring his console game design impulses through Lionhead's other projects, including Black & White Next-Generation, Project Ego, and BC.