Twitter Bootstrap Syntax

Question

Possible Duplicate:
What does the exclamation mark do before the function?

While looking through the Bootstrap plugins I see they encapulate their code with

!function ($) {
  // ....
} (window.jQuery);

rather than the

(function ($) {
  // ....
} (window.jQuery));

I'm more familiar with. Can some one explain the rationale behind this syntax. Thanks,