how to enable font aliasing

I've tried generating text using the cairo "toy" text API as well as 
using pangocairo.  However, I can't seem to get the text to use 
aliasing.  No matter what I do the text is drawn using discrete full 
black pixels.  I am using an image surface which I write to a PNG file 
for later examination.  Is there something obvious I am overlooking?  I 
noticed that when I draw circles the edges are aliased and lines are 
aliased depending on the line width and start/end points.  This is on 
Windows 7, by the way.
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