there are few solutions for this :
- select your curves in the graph editor and do a scale -1 on the time value.
or
if you use maya 4.0 :
with the trax editor create a warp curve on your clip and scale it -1 on the graph editor ( you keep your original curves intact with this method...)
or download on highend the scrip reverseClip on the animation section to reverse your clip in the trax editor...et voila...
gaviez