Ever had the problem installing any mpkg installer? Happened to me today, I received for no apparent reason information that JavaScript check failed during the iOS SDK installation. Was not able to resolve in a normal way, so had to hack a bit, here it is how (BTW works for any mpkg file, not only Xcode with iOS SDK).
1. extract the mpkg package with XAR (xar -xf path_to_your_installer.mpkg)
2. locate the dist file in the content subdirectory (iPhoneSDKSL.dist in this case)
3. open it in some text editor (vi will be fine in this case)
4. check the volume check function there and just change to JS code to return true:
function volumeCheck(){return true;}
function volumeCheck() { return true; }
5. close the text edit and save the file
6. repackage the mpkg back with XAR (xar -cf iOSDSDKInstall.mpkg path_to_the_extracted_location)
7. run the new package and all is well
BTW. if you use PathFinder app you can save lots of manual work, just right click the MPKG, select show content, then click the dist file and select open with and edit it in place. save it and that is it, PathFinder will repackage for you.
Happy hacking (in this case).