[Plugin: CSS3 Buttons] Any Support for Widgets?

Yes, see http://englishmike.net/2008/07/07/wordpress-quick-tips-3adding-a-shortcode-to-a-sidebar-widget/

Basically you add this to your functions.php

add_filter('widget_text', 'do_shortcode');

This will enable your theme (assuming it is widgetized) to run shortcodes.

Then create a text widget and put the shortcode for this plugin in it, as if you were using it on a post or page. I haven’t tested it but it looks like it should work.