Macrovision has redesigned the InstallScript engine for InstallShield 12 so that InstallScript code can now be compiled into a standard Type 1 (DLL) custom action. No need to pre-install the InstallScript engine anymore. Chris Painter calls this "the Holy Grail of Custom Actions". While this may be slightly exaggerated it is certainly something you should look at. As a side effect this design change may break your existing projects under certain conditions.

For more information:
Chris Painter's article about InstallShield 12
The InstallShield 12 public beta program