Yes. You will have a separate .dmg available for download when you go to the iOS developer X-Code download link.
Please note that X-Code 4.2 will only have an iOS 5 simulator and library . If you want to use a lower version simulator and a library with X-Code, just go to X.Code-> Prefrences-> Downloads Tab. On the Download tab, you will see two tabs:
1) Components . Here you will have the opportunity to download iOS 4.3 simulator (~ 600 MB), iOS 4.0-4.1 (~ 670 MB) Support for debugging devices, iOS 3.0-3.2.2 (686.3 MB) Support for debugging devices.
2) Documentation - Yoy will have the ability to download iOS 5.0, XCode 4.2 Developer libaray, Mac OSX 10.7 Core Library.

Install the components you need and there you go.
Hope this helps.
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