Subdivision Surfaces
Since version 3.0, Houdini supports Renderman Subdivision Surfaces.
So I did a test on my famous EarGuy model. It renders much smoother. The eyes get a little big though --->
Rendered as polygons
Rendered as Subdivision Surface

There is a texture mapping problem:

Show texture coordinates
(This shader draws white lines on the s and t coordinates applied in houdini.)
Posting this on the newsgroup comp.graphics.rendering.renderman helped me get the solution

When I did this test, in Houdini it was unfortunatley not possible to directly specify creases (edges that should not be smoothed out completely). Using a simpler model I put some in by hand:
Polygonal
Subdivided
Subdivided with creases.
The crease at the nose-tip is the most obvious, the mouth and eyebrows are also "sharpened".

This page is also availabe with smaller images

When I posted a question about the s and t coordinates being subdivided to comp.graphics.rendering.renderman, Guido Quaroni from Pixar explained it:

"s and t are variables of type varying. They are interpolated linearly over a subdiv face (like polygons).

The solution is to use variables of type vertex (e.g. vertex float niceS) and you will get a nice interpolation. I hope you can rename your variables and setting the right type in Houdini."

...and renaming variables and changing their type is easy in Houdini!

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