According to the Puget Sound Business Journal, Alaska Airlines and Delta Air Lines will announce a new "marketing alliance" on Monday.
Practically, what does this mean? Relatively little; Alaska already has codeshare agreements with both Delta and Northwest. As the most unique of America's seven legacy airlines, Alaska is also unlikely to be affected competitively by the new and larger post-merger Delta.
If you're planning to fly Alaska Airlines, though, do take advantage of its broad reciprocal agreements. Continental, American, Delta, and Northwest frequent flyers can all earn miles in their home programs on Alaska Airlines flights; check your particular program for details.