I believe the official number of additional new hires for the year is 80.
The only difference between DEN and ORD is that the DEN guys get paid for their commute to ORD. All of the flying is out of IAD and ORD - DEN trips don't see DEN except the first and last legs (for RJ, 146's spend all day going from DEN-EGE/ASE). I'd estimate there are 70-100 of us who live in DEN but commute to either ORD or ATL.
ATL and ORD are very similar for seniority. ORD trips are more commutable but ATL skeds are more days off. ATL is a small operation, only 10 airplanes, so not much variety in the flying.
IAD is likely to open up as a crew base (this straight from upper management).