OpenGL Transform Feedback has been worked on for months within the Mesa/Gallium3D world with progress having been made. Just a few weeks ago Stream-Out was enabled by default in R600 Gallium3D to allow for this support in the Radeon HD 2000/3000/4000/5000/6000 series open-source driver.
OpenGL Transform Feedback allows for keeping the post-transform rendering state of an object so that it can be submitted again by writing primitives processed by vertex/geometry shaders to buffer objects. Marek's latest work implements the GL_ARB_transform_feedback3 and GL_ARB_transform_feedback_instanced OpenGL extensions as needed by OpenGL 4.0.
GL_ARB_transform_feedback_instanced: "Multiple instances of geometry may be specified to the GL by calling functions such as DrawArraysInstanced and DrawElementsInstanced. Further, the results of a transform feedback operation may be returned to the GL by calling DrawTransformFeedback, or DrawTransformFeedbackStream. However, it is not presently possible to draw multiple instances of data transform feedback without using a query and the resulting round trip from server to client. This extension adds functionality to draw multiple instances of the result of a transform feedback operation."
GL_ARB_transform_feedback3: "This extension further extends the transform feedback capabilities provided by the EXT_transform_feedback, NV_transform_feedback, and NV_transform_feedback2 extensions. Those extensions provided a new transform feedback mode, where selected vertex attributes can be recorded to a buffer object for each primitive processed by the GL. This extension provides increased flexibility in how vertex attributes can be written to buffer objects. Previous extensions allowed applications to record a set of attributes interleaved into a single buffer object (interleaved mode) or to record into multiple objects, but with only a single attribute per buffer (separate mode). This extension extends interleaved mode to write into multiple buffers, with multiple attributes per buffer."
The good news is that for implementing these two transform feedback extensions doesn't require changes to the individual Mesa/Gallium3D drivers assuming they implement the internal interfaces. However, these latest patches from Marek doesn't introduce support for multiple vertex streams. Marek's also contributed matching Piglit regression tests for these new extensions.
Marek's ten patches for this support can be found on the Mesa-dev list and add in over 500 lines of new code.