That’s a funny one. I use motion alot and I can set it to any value fine. Are you using the latest build 2.71? If not download it and try the scene with it. I miss jenga.
Yeah, it’s weird, isn’t it? I am running 2.71, 64-bit on Windows 7 with 8GB of RAM, Core-i5 processor, and internal graphics processor (no fancy GPU, unfortunately). I don’t know if any of that makes a difference.
motion blur is broken for rigid bodies: https://developer.blender.org/T40017
it worked in 2.70a but not in 2.71 - so it is not a regression someone said -
that may be your problem, you can bake to keyframes until it is fixed
I couldn’t download your file, google was mumbling about 24 hours or something, but the link liero refers to must be it. Either get 2.70 alpha, or bake to keyframes.