It's looking more and more likely that any possible Cosmic Encounter expansion is definitely not in Fantasy Flight Games' immediate post-GenCon plans. The company has released a very specific list of release dates for September and October, and there's no mention of the game to be found. FFG is also currently releasing rules and news for a number of their games at a very rapid clip — just take a look at the front page of the website and you'll see what I mean — and, again, there's not a "Cosmic" or an "Encounter" to be found anywhere.
It's still possible that the expansion could be a stealth release, of course; there's no rule that says FFG or any other gaming company has to do things in any specific order or has to announce their every move. But I'm leaning more and more toward a late fall or early 2010 release, after this current huge wave of post-GenCon products has been in the pipeline for awhile.