The jQuery Curtain plugin allows you to place a darkened curtain over any DOM element, optionally with a graphic centered on the curtain. This can be used to disabled a portion of the page for any reason, such as to indicate an ajax call in progress.

Source on Github:
http://github.com/paulelliott/jquery-curtain/tree/master

Demo:
http://blog.codingfrontier.com/?page_id=157

It works by appending a div to the end of the body tag.  The div's absolute position and size are calculated on the fly to match the parent element.  This means that the curtain can be applied to any element type, regardless of its position.

Usage:

To apply the curtain:  $(".elements_to_curtain").curtain(options);

To remove the curtain: $(".elements_to_curtain").curtain("remove");

To get the curtain:    $(".elements_to_curtain").curtain("get");

To change the default options: $.curtainSetup(options);

Options (all are optional):

  loader_image - The location of the image you would like to overlay on the curtain.  If an empty string is used, no image will appear.
    default: '/images/ajax-loader.gif'