Is there a way in QGIS to create a "two-dimensional" color ramp for the graduated symbology; i.e. a color ramp that determines a color based on two different attributes? My situation is that I want to control the hue of the color of a polygon using one attribute and the saturation of the color using another attribute.
What you're describing should be perfectly possible using the new data-defined properties functionality in QGIS master (soon to be 2.0 release).
Nathan has posted this example of an "alpha by value" map which maps two attributes (unemployment rate and population) to color and alpha using the expression written in the upper right corner.
You can find a blog post describing the procedure at http://nathanw.net/2013/06/27/alpha-by-value-choropleth/