This is the hopefully correct second version of this post:
Softimage 2011 comes with a parser for mi files, which theoretically means,
you can use Mental Ray shaders written for other application
(for instance Maya and Max) almost "out of the box".
What you need are:
1. A "dll" file for your specific OS
2. A corresponding "mi" file
To get Softimage 2011 to recognize the custom shaders, you need to do the following:
• Go to your Softimage 2011 user directory and open the folder named "Application"
• In this "Application" folder create a folder named "bin" and a folder named "mi"
• In the “bin” folder create a "nt-x86-32" (
corrected,see comment below) folder
if you’re using a 32 bit Windows
or create a “nt-x86-64” folder if you’re using a 64 bit Windows
• Drop the "dll" file in the newly created "nt-x86-32" (
corrected, see comment below)
or “nt-x86-64” folder
• Drop the "mi" file in the newly created "mi" folder
• Start Softimage 2011
• The custom shader should have become available in the embedded preset manager of the RenderTree
Some additional notes:
1. Not every shader works "out of the box". Don't blame me!!!
2. The shader shows up in the Mental Ray section of the Preset Manager
3. This method doesn't create any user-friendly PPG, opening the shader gives you the bare parameters
4. You could also use the "bin" and the ''mi" folders in your installation folder, but to it’s probably best to keep the custom shaders separate from the factory shaders (so you do not accidently delete a factory shader while wanting to remove a custom shader…
PS - This is Windows centric, I know: I don't know if and how this relates to Linux
Be sure to tell us what brilliant gems of the past you have uncovered...
comment/edit: Stephen Blair 
points out in his Support Blog, the folder to put your dll's in should be "nt-86-32" if you're on a 32 bit SI2011. As I do not have access to a Windows 32 PC anymore, I erroneously assumed that folder to be "nt-x86". Thanks to Stephen Blair for pointing this out.
