Re: Black edges around objects when I enable motion blur!? Help!

Maya (as well as most other renderers) always outputs un-premultiplied images when rendering. You can confirm this by setting the background color on the persp camera to white and rendering. The color channel will be white, but the alpha channel will be black. Compositing software can handle this quite well, but the 2d motion blur algorithm cannot. Which is why its usually best to render 2d motion blur over a pure background (meaning black).

One question I have is about the white background in your render, is that actual geometry that is colored white, perhaps with a surface shader, or is it a background color set to white? If its actual geometry, that can also cause very bad smearing in the color channel. Maya's 2d motion blur technique, as well as most other 2d motion blur technology, does not handle objects crossing other objects as well as they do objects moving across blank space.

Whatever the setup, the other posts are on the money. Render 2d motion blur passes over a pure empty background for the best quality. Combine this rendered pass on top of any backgrounds in a compositing package.

I also advise against using Smooth Color option in the 2d blur controls and suggest keeping it on Smooth Alpha, mainly because using smooth color tends to blur the color channels far too much, even at a smooth value of 0.