Shouldn't a bgiframe be applied to popup jquery ui dialogs (dialog, datepicker) all the time instead of just for ie6?  Java applets, for example, do the same thing in all versions of IE as selects do in IE6.

 

See the attachment.  It shows a jquery.ui.dialog (on the left) being covered by a java applet (on the right).

 

If you apply a bgiframe - which I did using a modified version of jquery.ui.bgiframe.js - the entire dialog is displayed correctly.