In exploring the settings of the Oren Nayar shader in the Blender internal rendering engine, I find that there is an upper limit of 3.14 for roughness.
Is this the constant Pi?
Why is this the upper limit?
How does this upper limit affect the shader?
[NOTE: if the Wikipedia article Oren Nayar reflectance model includes an explanation that answers these questions, I don't find it. Also note that the current manual does not include any information that there is an upper limit.]