Delta and Northwest Airlines are merging, and right now you can go ahead and transfer frequent flier miles between the Northwest WorldPerks and Delta SkyMiles programs with no fees. You can even move them back and forth as you like. This is nice if you don’t have enough of either individually to get an award, but after combining you do. Also, if you link your Northwest and Delta accounts by May 31st, you’ll get 500 bonus miles.
According to this timeline, they will eventually all be merged into SkyMiles anyway in December 2009.