Wanted to add (in case the original poster is unaware) that you can redeem Delta or Northwest miles (can't combine from the 2 programs though) for travel on either one OR a combination of the 2.
This means that you can redeem either WorldPerks or SkyMiles for a domestic award mixing 2 or more partner airlines, for example:
Outbound
DL TLH-ATL-IAH
CO IAH-SEA
Return
NW SEA-DTW
DL DTW-ATL-TLH
All for 25,000 miles in coach... as long as standard award seats are available on all of the flights :-: