I'm very much playing a wait and see approach with this, as there are many reports on the forums of people running the 64-bit client and having issues.
Even though Blizz told them expressly NOT to run the 64-bit client.

That said, I am unsure from my forum trolling if one has to download it separately or if it came with the patch and is merely a second .exe in the WoW folder to click. Though judging from the tales I've read on the forums, it is or was something you download separately.

I'm perfectly fine waiting for Blizzard to say "It is now live, here is where you download it and what you do to activate it properly" and until such time I advise anyone else to do the same.

For the record though, we lost one of our core raiders last night due to him fooling with this despite the warnings, and he had to run the repair tool and go through the optimization process about 4 times before it would let him in. Between all that and actually patching, he missed raid. Oh well, better for him we were finishing a Legendary stick for someone in Firelands last night before we went into DS so he didn't miss much.