Autodesk just released a revision to Maya 2009 Service Pack 1, called SP1a. It addresses a few issues in the current service pack, but will require you to uninstall your existing SP1 and install this update.
Important: Maya 2009 Service Pack 1 has recently been updated
If you installed it before April 16th, 2009 , you must uninstall the existing version, then download and install this new version. The new service pack includes the following important fixes for you:
* mia_material_x_passes shader and shadows
Previously, the mia_material_x_passes shader caused shadows to behave incorrectly. This has been fixed.
* mental ray for Maya and processor limits
Previously, the mental ray for Maya renderer was not using all available processors when hyperthreading was turned on. This has been corrected so that all available processors are used for rendering.
* Color per vertex shading
Due to an optimization in CG 2.0, color per vertex shading was incorrectly rendered in the Render View and in batch rendering. This has been fixed.
* Stereo cameras and ATI graphics cards
Due to an update in CG 2.1, stereo cameras in Maya caused display errors when using an ATI graphics card. This has been fixed.
I’m not real sure why they didn’t just release this as a SP2 that will apply on-top of SP1.
via Autodesk – Autodesk Maya Services & Support – Autodesk Maya 2009 Service Pack 1.
The reason that we did not provide a small patch update for Maya SP1 is quite simple: Maya Service Packs are always full installs. It makes tracking which version you’re running a lot easier.