One of my colleagues Ed, has come up with a really useful PHP script that makes it trivial to concatenate and version css or js files on your site. What it does is takes an array of the files you wish to join together and it will create a versioned include. When you update your source files the script automatically re-builds the combined file and adds a new version number to prevent caching of outdated files. Not only that it also maintains a history of old versions for compatibility with code that uses the older file versions. See Ed's post for more details