This announcement is not good news for United. I think that you may possibly see United counter-offer for Delta. Why? Simple. A merger with Continental cannot be easily consummated due to the "golden share" provision that Northwest has over Continental. Are there ways around that? Yes, but it's tricky. And with Continental not being a bankrupt carrier, it would be difficult to right-size the merged carrier and shed inefficient fleets, facilities, and excess employees. That makes a merger much more expensive and difficult to fully accomplish. I don't think United can afford to let US Airways merge with Delta. It would push them down in the pecking order. And the loss of US Airways from the STAR Alliance would cost United A LOT of revenue (even though it's not a foregone conclusion that a combined US/DL would leave STAR, but I consider it likely).
First let me say that I am not in favor of mergers because of the havoc they wreak on front-line employees, who are always the ones left standing when the music stops. But, I'll be honest in saying that in the long-term, consolidation is going to happen in the industry. It's only a question of who fires the first shot. And this could very well be that shot. We all know that this is an industry ruled by executives who are not known for fiscal restraint and sound economic decisions. If this US/DL merger goes forward and US is successful in convincing Delta's creditors and shareholders that this is a favorable deal to them, it will cause panic among the rest of the industry. I'm quite sure NW is already talking with SOMEONE. My guess would be either Continental or American. United knows this. If US succeeds in acquiring Delta, and then another competitor makes a move for NW, that will leave United very vulnerable.
So, based upon that, and the much-stated quest of current UA Senior Management to merge with SOMEONE, I would expect either a counter-offer for Delta as a whole, or for certain strategic assets.