The key is when the beta goes up. They don't need the test servers for anything past ICC bug fixes, so I would expect the beta to go up Jan/Feb, the usual 3 months of testing and several waves of invite keys getting sent out, and a summer release. One of the Devs at Blizzcon said that by next year's Blizzcon, everyone will be sick of Goblin Rogues, and that's long before November.

However, it is entirely possible that they bit off more than they can handle, and it will be delayed, and they may have some content to pull out to give everyone something to do - it would be VERY cool if they did The Old Kingdom, but it would pull devs from the expansion. (The Isle, as it was said, was always planned, they just had to change the release dates - there was a massive amount of work in that patch, no way they pulled that out at the last moment).

I do not buy a year before we see new content. They're busy at work, no doubt, and while there's no word of any alpha going on, I'm betting on Jan/Feb for the beta - they'll have a nice long stretch with no holidays to get in the way to crank out the code and art and files.

GameStop did the same thing for Wrath, and for BC. They're just getting free money from the interest on your pre-order. They do NOT have inside info, they're guessing, like someone else said, and they're just making room in their system for it. You'll know when Cata is coming, when the first wave of beta keys go out - until then, it's all speculation.