Just installed XCode 4.3.1 (4E1019) and as expected, it comes with iPad Retina (a.k.a. iPad 3, the "new" iPad) simulator, YES!
Unfortunately the resolution is just too high (I think about 1536x2048 in portrait and 2048x1536 in landscape) for my 13" MacBook Pro, even if I adjusted the scale to Max of 50%, it's still too big for the screen... I wish apple can also provide another 25% or 10% scale, otherwise developers shouldn't buy 13" MacBook Pro or Mac Air as the screen is too small....
As it's installed under "Application" instead of "Developer" as in version 4.2, it took me a while to find it.
Also noticed it only comes with 5.1 simulator, you have to install the 4.3 and 5.0 simulators separately.
As it requires OS X 10.7.3, make sure you upgrade your Mac OS to the latest version.
Any way, surely got a few more things to try and extra huge images to create...
[Updated 9/3/2012 with images and screen resolution info added]