Mookou asks for a "51-point talent preview" before the expansion, and while we probably won't actually get that, what we'll get is likely the same we got with the Burning Crusade release -- about a month before the expansion, there'll be a huge downloadable content patch for everyone (we've heard before that it will be patch 3.0) that will contain all of the updates everyone is getting: all of the talent updates, game mechanic changes, model updates, and so on that we're going to see in the expansion. Blizzard will make all the changes and fixes around that patch, and then when Wrath is actually released, you'll just be installing Northrend content, and the game itself will be (supposedly) ready for all of it to work together. And Neth confirms that this is probably the plan -- it should work much like last time around, though the patch will probably be closer to release day.
Now, Blizzard is much better at this than they used to be, so we'd imagine that (hopefully) things will go a little smoother this time around, and we won't have nearly as many problems or hotfixes as we saw last expansion. But then again, we do know they are planning things a little differently this time around -- they've divided the entry and exit zones up, so we won't see a big mess like we saw in Burning Steppes or Hellfire Peninsula. And there are rumors that we'll see a World Event go down before we actually go to Northrend, and as of right now, we don't know if there is going to be a testing phase for that or how long it might take. Last time around, too, you'll note, everyone got a free respec (and some even got two), and we haven't heard anything about that this time yet.
Of course, as always with Blizzard, time will tell. Given that we're expecting a November release, that would put a big 3.0 patch in October, and we already know Brewfest is going to have some changes then, so 3.0 could come in conjunction with those changes or around them. We'll have to wait and see.