The first thing I would do is provide a fallback colour for your nav lists. IE has no idea about ‘rgba()’. Declare a similar ‘background-color’ colour (a bit pinkish) before the ‘rgba()’ and IE will then at least display the solid pink colour. You could also make a transparant ‘*.png’ and use that as a background.
!Edit: So while I was typing this my IE8 was displaying the menu. Now it isn’t!! and I can’t get it back. Sorry can’t help anymore :-(