I found this out trying to install XCode 4.2 on OS X 10.6
Failed install preflight: Error Domain=PKInstallErrorDomain Code=102 UserInfo=0x1d9a30 “The package “iPhoneSDKTools.pkg” is untrusted.” Underlying Error=(Error Domain=NSOSStatusErrorDomain Code=-21 47409654 UserInfo=0x1dbe90 “The operation couldn’t be completed. CSSMERR_TP_CERT_EXPIRED”)
Yep turns out XCode 4.2 (and probably many others) will actually expire. So turn off the automatic clock sync, and set your time back to before the package was issued (Xcode 4.3, released on February 16, 2012..) and you’ll be good to go.