Here's a trick that I found on an old cgtalk post...
Sometimes you have a character's hand constrained to a surface and in 1 frame you switch off the constraint. Well, since the renderer reads past and previous frames for motion blur you wind up with a crazy streaked motion blur on the frame where you turn the constraint off.
1. Grab the trans keyframes of your object (or say your character's arm control) on the frame BEFORE the constraint gets turned off.
2. Turn off snapping in your graph editor and drag the keys extremely close to the next keyframe (to the right) . Zoom in and drag them as close to the keyframe where the constraint turns off as you can. Voila, proper motion blur.