I'm working on a new version of a plugin, and would like to use WP 2.8's new widget API. Without additional measures, this would break backward compatibility. I'm not sure how many people would be affected by this, but this could be a potential support nightmare.
Does the automatic plugin upgrade feature check the version info in the readme.txt and warn users if the new version of the plugin no longer supports their older WP version? Does it upgrade if users ignore this warning?