I’ve had the same problem for a while, and upgrading to 3.2.6 did not help. This does not happen from the ‘plugin’ page, where it sees the plugin is up-to-date, but pops up on the settings page for this plugin.
There is a new version of Google XML Sitemaps available. Download version 3.2.4 here or upgrade automatically.
I’m running WP 3.3.1