Resetting the clock also works to re-install XCode 4.2 on Snow Leopard.
I did an OS reinstall after a drive reformat and XCode wouldn't install from the Disk image I'd used earlier.
"Unknown installation error".
Reset the clock from 2013 back to 2012 and it installed no issues.
Might be good if Apple could re-package the installer with an updated certificate...
I noticed the error was coming from an expired certificate on only the iOS portion of the install. If you don't care about that (I was only interesting in building universal binaries that can run on both PPC and Intel Macs), you can try declining the second licence agreement covering iOS. Then the installer will proceed with only Mac-related files which, for me at least, was ideal.