How to monitor several sites for updates from one central site?
You can use Drupal Remote Dashboard module for this


  This module is for system administrators. It's a tool to monitor and manage any number of Drupal installations at once and it's built to be extensible by other modules. Since version 2.X this module also comes with the highest level of security where as all communication between the dashboard and each of the cores is AES encrypted, hence the dependency (see below). Optionally the monitoring data will be made visible in nice graphs if the Google Chart API is available.