We know that
-The release date will be announced at least 2 months before the release
-The last 2 expansions, the pre-patch PTR has started at least 1 month before the pre-patch goes live
-They have said they plan to run a 4-6 week pre-patch.
-The choppers September 30 deadline makes it unlikely that 6.0 can be released before September 30 (and probably before October 7).
It means that anything before October 28 is all but ruled out, because a September 30 6.0 launch would only give a 4 week pre-patch period to October 28. It also means that November 18 is the most likely date because (1) I don't think they are likely to release 9 days (or 2 days) before Blizzcon (2) They also won't likely release on Veteran's Day