All of the WordPress.org Plugin pages show "Compatible up to:" version number on the right side (i.e. http://wordpress.org/extend/plugins/PLUGIN/ )
It would be awesome if on the Install Plugins page -- http://DOMAIN/wp-admin/plugin-install.php -- there was a checkbox saying something like "Show only Plugins compatible with your version".
The data is already there at wordpress.org and I'm hoping it would be easy to query and sort against.
I'd be willing to help test such a feature.
Let me know. Thanks!