Dr. Ed Catmull says that the team will not be limiting the free version of the software in any way, but users must agree to use it for non-commercial purposes only.
Check out more information on the pricing changes and the free release here.
For those of you who don't know about RenderMan®, or don't know about the 3D graphic process, you can read a bit about what it is on Pixar's RenderMan® website.
RenderMan® is a registered trademark of Pixar.