Ha, incidentally, I posted a more in-depth look at the same issue of overincluding css and js by plugin authors at almost exactly the same time as you – http://beerpla.net/2010/01/13/wordpress-plugin-development-how-to-include-css-and-javascript-conditionally-and-only-when-needed-by-the-posts/

Also, I’d be careful about your solution – it seems to assume a single post page only – what about galleries on the front page. Not everyone uses excerpts.