I discovered a clever method for separating the case halves of a Spice unit, after the techniques listed in MoHPC failed to work for me. I have photographed the technique, but have not yet written up an article for the tech archives.
Tie a loop of waxed dental floss. After removing the two screws in the battery compartment, place the calc face down and spread the halves apart at the top no more than an inch (watch the battery terminals). Insert the floss between the halves and slide it past all six tabs (three each side) and around the corners at the bottom so that the floss exits through the case seam at two places on the bottom edge. Internally, the floss should wrap around the long hooked bridge attached to the bottom case half.
Hold the main (top) case half in place on the table face down (use a rubber glove if you like) and gently pull the loop of floss away from the calc in order to detach the smaller back case half.
I've used this technique three times without damaging a case.