Rey, here's a "pluginized" version of your code that can be called like $('#foo').fillbox() or $('#bar').fillbox({ url: 'foo.php' }). I've found that, for element-specific code, it's not a lot of extra effort to take the typical "pass the element as an argument" version and pluginize it so that it operates on a jQuery collection of elements, like built-in jQuery methods. Also, note how my version has default options that can be changed globally, as well as overridden on a per-use basis.

http://gist.github.com/465622