Here is how you should be updating your plugin. The WordPress install sends data to the repository and the repository sorts out the versioning and talks back to the install. Here is an article by Mark Jaquith on this subject, and the lengths to go though to turn it off.
There has been a vigorous discussion going on regarding what data
WordPress installs send to WordPress.org when doing update checks.
Because WordPress (the software) doesn’t know whether a theme or
plugin is listed in the WordPress.org repositories, it has to check
them all, and let the repository sort it out. Some have expressed
concern that private plugins developed for a single client could
contain sensitive information in their headers, like contact
information for the developer, etc.