Use get_stylesheet_uri()

[justintadlock]

The primary stylesheet URI for the theme should utilize the get_stylesheet_uri() function. Some plugins (and even child themes) might rely on the get_stylesheet_uri filter hook.

If, for some reason, you don't want the style.css file to load, you can filter get_stylesheet_uri to return a file of your choice. The important part is that the hook is executed.

[digisavvy]

Do you mean get_stylesheet_directory_uri()? I'm assuming as much.

[justintadlock]

No, I mean get_stylesheet_uri(), which is a different function. It's the function for loading the theme stylesheet. get_stylesheet_directory_uri() is for referencing the stylesheet directory.

Reference: http://codex.wordpress.org/Function_Reference/get_stylesheet_uri