It seems the leaks are flowing from White Dwarf's like crazy atm. A codex Legion of the Damned is also leaking, with pics to show that its real. This is going to be a little crazy in March with two mini-dex's hitting.
Here are some links to the leaked images. The guys at 40k radio apparently have a first class link to early White Dwarf images.
Of course we dont know much yet, except that there are rules to use them as allies for any imperial army, and specific scenarios as well.