Google Page Speed recommends adding a Cache-Control:private setting to static files like .js and .css while adding a Cache-Control:public setting to images. I implemented a quick fix for it that lets you configure what file types receive the private heading and defaults all others to public. I honestly don't have a clue how to work GitHub, so I'll just paste what I did.
//Specify which file types will receive a "Cache-Control: private" header. All other types will be set to public
$settings['cachePrivate'] = array('js','css');
else header('Cache-Control: public');
$parts = explode(".",$cachedFile);
$fileType = $parts[count($parts)-2];
My line numbers may be off a bit but I'm sure you get the idea. I'm also sure there's a more efficient way to do this, but I was in a hurry.
This should be pulled, sounds nice!
Also, another tip for proxy caching is to not use ? in the URLs you want to be cached.
I know the .htaccess file does this, but maybe a line in the docs would help, seeing as you have to edit the path to SmartOptimizer inside said .htaccess.