Interesting, but surally the reporting of an unassigne variable only yeilds a warning not an error? I this is the case the service pack won't break an existing build, just report more warning that it used to. I think that this is an acceptable change to be introduced via a service pack.

Of course it might break you're build if you're using the treat warnings as error switch, but you can always turn that off. (Or just fix your code).