If I assign a user an MSI package, what initiates the installation, is it a logon or a reboot?


Actually it is neither. Programs are automatically downloaded and installed when they are first run by the user. The download/installation can be triggered by either the user clicking on the application icon located in the Start Menu which appears once the GPO is created, or if the user opens up a file that is matched to that application. For instance, the opening of a Word File would initiate the installation of Microsoft Word. Jeremy discusses the assigning of applications on Chapter 11 of his book. (Third edition, with the gear cover.)