This should be a core feature, not a plugin, but alas, here it is.
The aim of the plugin is to help people keep up to date with new wordpress version releases.
The plugin takes a simple approach to get the users attention. Once activated it checks an XML-RPC webservice for update news displaying a message at the top of every page in the wordpress admin user-interface. It will check for an update to the message every 15 mins with an additional check being kicked off if the installed wordpress version changes so as to give instant feedback on upgrades.
Grab it from Peter Westwood’s Site: http://blog.ftwr.co.uk/