I’m fairly new to Blender myself.
It’s usually regarded as bad practice, but…
This might be one of the rare cases where triangles are acceptable.
If your model is going to be static (not rigged and animated), and you are not going to put a subsurface modifier on it, triangles shouldn’t cause too much trouble.
You could select these two vertices.
Press “j” (in version 2.63)
and it will divide the face into two flat triangular polygons.
I know there are of people that will say never to use triangles, but I think there are times when you can get away with it. It just depends on what the model is going to be used for.
Just my suggestion