Ok since the other thread is a little misleading.
None of this is official. Until you hear it out of the creators them self.
Here are my thoughts.
(Before you go off saying i don't know what i am talking about not true i have worked for gaming companies in the past and know how a lot of the process works)
PTR - 4.0.1 Yes its buggy and there are problems. This is not the final patch build, Blizzard already has this and it is complete. How you say? Internal testers, before the patch even hits us people have already tested it they simply put it on PTR for people to test out how it will react in a bigger population of gamers. The bugs you find are fixed already there done.
OK so that ends 4.0.1.
Beta Build - People who are saying its still buggy, it's still messed up, think about how long it was on Alpha testing.... a long time. They sent out random invites to test the game on a bigger server with more people and to test out how the downloading goes. Trust me when i say this is no where near the final build blizzard has right now. The version they have is far ahead of the one on beta.
Cataclysm is done guys its finished with maybe a few minor issues that can be fixed internally. Cataclysm will not come out perfect there will be bugs and that's what patches are for.
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My final saying is
4.0.1 - Oct 5th
4.0.3 - Week before Cata
Cataclysm - Nov 16th at the latest