You can "Do the Dew" without resistive sensors.

A clean glass plate is a specular reflector, unless covered with dew (or
dirt). Then it becomes a diffuse reflector.  A diffuse reflector scatters
light in all directions (like paper). A tight IR beam can be arrainged to
bounce off the glass, and miss the target, unless there is something on the
glass.
--
Where's dave? http://www.findu.com/cgi-bin/find.cgi?kc6ete-9

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.