Same thing happened to me, grey background and no text or progress.

Using bootstrap 3.1.1 and MVC.NET

Fixed the following -
added modal-dialog and modal-content div containers
removed the 'hide' from the outer 'modal hide' div class
replaced the 'bar' with 'progress-bar'

So the new line should be:

      var pleaseWaitDiv = $('<div class="modal" id="pleaseWaitDialog" data-backdrop="static" data-keyboard="false"><div class="modal-dialog"><div class="modal-content"><div class="modal-header"><h1>Processing...</h1></div><div class="modal-body"><div class="progress progress-striped active"><div class="progress-bar" style="width: 100%;"></div></div></div></div></div></div>');

I use it on a form so i added to my button the following onclick attribute

onclick="myApp.showPleaseWait(); return true;"