According to ZeniMax website, Arkane Studios is working on a game using the Unreal Engine.
Go here http://jobs.zenimax.com/index.php?a=search
And just tick the Arkane Studios box. Click Find Jobs.
Read the bottom part.
Another interesting note, check the Bethesda Game Studios jobs. You can see a Quest Designer job here. I think someone already pointed this out. Just a detail.
And at the bottom, Game Programmer job, with this note :
Bethesda Game Studios is looking for talented programmers to join our team that is pushing the bleeding-edge of RPG development for the PC and future-generation consoles.
edit : And don't forget to click the "MORE INFO" button, some nice bits, seems to tell us they will keep using the same editing tools for the upcoming game (TESCS and GECK)