Well it appears that Delta and US Airways have given up on attempting to slot swaps at LaGuardia and Reagan National. The deal was originally announced almost a year ago and has been the subject of heavy scrutiny by the government and other airlines. The deal was also being fought by US Airways pilots union as it was going to mean the closure of another base.
While both airlines attempted to make some concessions in hopes of getting approval, they weren't enough for the regulatory bodies and thus both airlines have abandoned the deal.
So what does this mean for the airlines? Delta will have to organically grow at LaGuardia and look for opportunities for reducing other frequences to serve new destinations. US Airways will have to figure out what to do with all of its slots. Currently, US has 25 flights between Philadelphia and LaGuardia. There is no reason they need this many flights but they have them to keep the slots active so that they don't lose them.
The new few months should be interesting to see how US Airways handles their LaGuardia operation and to a larger scale their entire operation.