ATL was doing well in the summer, so I'd say they were making money on one hub, the whole reason it was dropped was due to it's poor performance in the winter. Like I said, ever since it had been dropped DL has been considering bringing it back summer only, and now it's on the smaller B752 I can see this doing well although given how much cargo the B763 was carrying, the drop in cargo capacity might hurt a bit. JFK will carry much more O&D traffic obviously but is going to face an uphill struggle with CO 2x daily on EWR, however in the summer I think these routes will do fine, as long as DL market them better than they did last time!
I got the figure from here-
http://www.airliners.net/info/stats.main?id=101
Couldn't comment on it's accuracy, but the fact that NW and DL have routes longer than CO on their PW B752s suggests it could be true. These are some of the longest B752 routes:
NW DTW-FRA 3616nm
DL CVG-AMS 3599nm
DL ATL-EDI 3484nm
CO EWR-TXL 3458nm
As you can see, the airlines with the PW powered variants seem to be pushing them furthur, and CO are very reluctant to go any furthur than TXL as that already takes pretty big payload restrictions Westbound.
DL's are the recently acquired TWA aircraft so you could be right there, not sure about NW though.
Running days 135 DL197 1050-1440
It's only June it operates on days 1,3,5, from July onwards it's daily.