Xcode 4.2 Snow Leopard Doesn't Install
Just change date to January 2012 or something, it will install just fine. The certificate expired in March.

Updated, steps:


turn off automatic synchronization of date &amp; time  
change actual date of the system to 01.01.2012 (for example, prior to
26.03.2012) 
install the Xcode 
turn the automatic synchronization back on.