Honestly, these merger rumors are constantly buzzing around Alaska. There were rumors of Kelly flying to Atlanta, Delta pilots seen at Alaska flight simulators...all this year.
AS has quite some number of airlines that are interested (AA, CO, DL, NW). From a stand-point of fleet, CO, AA, and DL make the most sense. As far as route system is considered, I agree that DL is a great match, but I think any of the airlines I mentioned could benefit. If they wanted to expand routes to Asia, Alaska/Horizon could provide great feed for those flights.
If any external entity owns more than 20% of Alaska's stock, a poison pill is triggered, which makes aquisition of the remaining shares prohibitively expensive.
Alaska is still in good financial health, and so I hardly see them in the position RenoAir was in near the end. Also, I hope that they do stick around, because they are not just another airline. They have a really unique history and route system...they have always beat to a different drum. Also, I've been impressed with how a relatively small carrier has been widely regarded as a high-tech leader among airlines.