I swear there's a ticket for this already, but I can't find it.
Without grabbing any extra information from wordpress.org, WordPress can be made to determine if a user's version of WordPress is out of date given the following assumption: That the WordPress Development Blog is running the current stable version of WordPress (this may be a bad assumption - as of now it is running an alpha version).
Adding a nicer (markup/CSS) version of
$latest_version = parse_url($rss->channel['generator']);
parse_str($latest_version['query'], $latest_version);
$latest_version = (float) $latest_version['v'];
if ( -1 == version_compare(get_bloginfo('version'), $latest_version) )
echo 'A newer version of WordPress is available. Please upgrade.';
below the dev blog rss fetch would do the trick (type casting as float may or may not be deemed necessary).
wordpress.org could even be set up to make clever links in the above upgrade notice easy. http://wordpress.org/development/version/$version_number (or something similar) could point to that release's permalink.