OK, This is an inspired hack. I haven’t tried it, but it’s so crazy it might just work…
Make your rock in several pieces and give each piece the same point of origin. (ie, stick the 3D cursor in the middle of them and click “center Cursor” in the mesh editing buttons for each rock bit). Make an empty at this location too.
Now comes the fun bit that i’m not certain about, so look for tutorials…
Use the Physics Engine and record to IPO function to record the animation of all the bits breaking up and going everywhere.
Now go back… Remember the empty? At the beginning of the animation, set a key-frame for it’s location. Now go forward to the frame where the rock just touches the ground, move the empty to about the middle of the rock and set another key-frame. on the empty.
OK - For the magic… For each rock bit, set a “Copy Location” constraint pointing to the empty. At the first frame, set the influence to 1 and create a key-frame (Press I-key while pointer is over the constraint window to set a constraint keyframe1)
Move to the frame where the rock touches the ground. Set another constraint keyframe at 1
Move forward about 5 frames, set the influence to 0 and keyframe it.
Go back to the beginning and hit alt-A with the pointer in the 3D window to see the result.