For me, the first real impact of the DL/US slot swap has come to pass: last DCA-MSN nonstop appears to be January 31, 2012. Also, it appears that US won't be replacing it, as they're focusing their new DCA nonstops mostly in the south, with a few in the northeast and one to OMA. See this
LINK. Not surprising, though, as US doesn't serve MSN at all, so it probably wasn't worth investing the money in start-up costs.
That said, there's been a NW/DL DCA/MSN nonstop since about 2002 or 2003, so apparently it was profitable. Additionally, since I moved to DC from MSN in early 2007, every time I've flown the nonstop over the last five years they seemed to be 80%-plus full; fares had also steadily trended upward to the point where it seemed the minimum I paid in 2011 was about $300ish compared to about $225ish only 2-3 years ago; DL had also upgauged the metal to a CR9 or CR7 from the long-standing CRJs.
On a related note, it will be interesting to see how DL changes its schedule at DCA as a result of their significantly reduced slots.