Thank you for the reply, Falko. Much appreciated.
Did you have a chance to look at the link that I cited in my initial post? There is a fundamental problem with using "install" over "upgrade". In short, with "install", there is the potential for packages to be removed and never reinstalled.
In the case of Apache, using 'apt-get install -y apache2' will cause libapache2-mod-php5 to be removed (for reasons explained in the link above). Needless to say, when the package installation script attempts to start Apache after the upgrade (or installation, in this case), Apache fails to start due to the presence of Mod-PHP-specific configuration directives. The only way to reconcile the problem is to reinstall libapache2-mod-php5 manually.
If any other ideas come to mind, I'd be grateful to hear them.