Update: strict mode is now supported in the compiler.
Just use --language_in=ECMASCRIPT5_STRICT.
References:
http://code.google.com/p/closure-compiler/issues/detail?id=69
http://code.google.com/p/closure-compiler/source/detail?r=873
http://code.google.com/p/closure-compiler/source/detail?r=1114