Winter '12 Force.com Platform Release
New IsTest Annotation

You can specify which test methods get executed during the installation of a managed package by annotating test methods or an entire test class with the IsTest(OnInstall=true) annotation. A test method or a class that doesn't have this annotation or that is annotated with isTest(OnInstall=false) or isTest, won't be executed during installation.

This annotation applies only to managed packages created in the Winter '12 release or later. Packages created prior to the Winter '12 release aren't affected by this annotation and all their tests run during installation.