At this point when I activate a plugin and an update is available it updates the plugin. I like to create a plugin myself that I want to use private, I have several WordPress blogs and if I made an update to the plugin I like those several WordPress blogs to state that there is an update available. Does anyone know how to do that?
There is a plugin for that: Automatic Updates For Private And Commercial Plugins.
To prevent upgrade checks against the WordPress.org data base in your plugin use the function Mark Jaquith has written.
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Note that update check requests were changed to use JSON and Jaquith's functions no longer work as-is on current WP version. – Rarst Jan 11 '14 at 17:57