I predict the Cataclysm release date will be Tuesday October 26th. Everybody says the date will be November 2nd, but there is one glaring piece of broken evidence.
The following information was dataminded on the 18th of September.
- Deathy - Right Click to summon and dismiss Deathy. / Instant
This is the BlizzCon companion pet. The pet information hasn't been data mined in patch 4.0 yet, but rather from the Cataclysm beta. This leads me to believe the 26th will be the release date, as it would be the Tuesday following BlizzCon, which means attendee's will be able to activate their pet shortly after the convention. Most item's distributed at past BlizzCon's were able to be activated immediately, and the last thing Blizzard is going to do is piss off their fan base dedicated to come to a conversion based around their products.