A UA/CO merger is a reasonable speculation. Another snippet from today's WSJ article reads:

If Delta and Northwest proceed and the transaction receives regulatory approval, the combined carrier would be the largest in the world by traffic. Such a merger, or even the prospect of one, is expected to set off a wave of consolidation, with UAL Corp.'s United Airlines and Continental Airlines Inc. seen as the next possible pairing. United has been an avid proponent of mergers. Continental has said it would like to remain independent but would act quickly if the competitive landscape changed.