Highlight the range you want, go to Format -> Conditional Formating
in the first box change "Cell value is" to "Formula is" then put this formula in the box
=IF(AND(($G$11=$H$11),($G$11 = "")),1,0)
then on format button choose No Fill
Just a quick follow-up to Dave's advice, if you want the conditional formatting for an area larger than the two cells, you will have to adjust the relative cell refernces (i.e. $G11 or G$11) to make sure that the entire selected area is appropriately formatted.
old, slow, and confused
but at least I'm inconsistent!