Here's a quick render of an Edge_Blend node controlling Transparency. By adjusting the Attenuation value on the node, you can control the transition rate between the two colour values that you specify for 0 and 90 degrees incidence. I just used white (opaque) and black (transparent) in this example, with Attenuation = 1.

(edit) oops, the Edge_Blend node was also controlling diffuse colour in this image, not just transparency.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)