As an almost universal rule of thumb, there is no perfect simulator/emulator substitute for a target hardware platform.
For example, the iOS Simulator runs in an x86 environment. There's no way to throttle it to run at hardware spec speeds, never mind that the Safari in Simulator is quite possibly running different code than the Safari that ships on the devices themselves.
I had never heard of the iOS simulator! Thanks for mentioning it, it just changed my friggin life. I'm on Snow Leopard, and there is no free upgrade to Xcode 4.0. I hunted down a torrent of 4.2 and got it installed though, and the simulator is pretty great.
My larger point is that the hassle isn't supporting X version of a browser anymore, it's supporting X version of a browser on various hardware devices. Web-dev hell isn't going to be the fault of IE anymore, it's going to be the fault of mobile platforms with differing browsers, screen-sizes, and use patterns.