Free on 5 June 2012, for those with Dungeons of Dredmor. 4 new skills (Clockwork Knight, Rogue Scientist, Battle Geology, Warlockery), new enemies & items as well as Steam Workshop mod functionality.

Details/screenshots here on the steam page - You Have To Name The Expansion Pack.

I've been playing it all day and really like the new stuff. Though I got it with the other DLC so I'm not entire sure (other than the skills) as to what comes from what. The mods that are already up in the Steam Workshop are pretty awesome as well.
 
Also I did a little First Impressions video.
  

You can actually name the expansion. On the main menu if you click the 'You Have to Name the Expansion' subtitle you can edit the text. Gives you a Steam achievement, and we all know those are serious business.

Main menu, not the config tool.

Edit: Dungeons of Dredmor - The Revengencing

I named my expansion: Dungeons of Dreadmor: Year of The Bow Edition.

I love dungeons of dredmore so much. Kinda feel rotten about only having paid like €2 for the game and expansion pack.

Thanks to the other post here about naming it, went with 'Tomb of the Burger Lords'. Too bad other games don't do this as I love the chance to name parts of the games on whatever feels right at the time.