I pulled down the example BLEND file that demonstrates how to explode a cube.
I thought the time IPO could be used to reverse the explosion and have the particles fly from off screen and form the cube? But this does not appear to work correctly. The particle cache gets all messed up.
Can someone take a look at this BLEND file?
Press play and you will se the explosion works fine.
Then go to the IPO window and add a time IPO in reverse, from 100 to 0 and you will see the explosion no longer works. I even tried clearing the cache and deleting the cache.
Exploding_Cube.blend (431 KB)