Popular softwares include an automatic update checking mechanism, that will notice the user when a new version of the software is available.
Piwik should notice the user, in the user interface, that a new version is available.
In the process of checking the updates, we would learn more about the Piwik community: number of users, frequency of stats consultation, breakdown by version, country, etc.
This feature is critical on a security point of view: it would be the only way that we have to notice all the users.
Now available in 0.2.30!