EDIT: I no longer recommend doing this step!

It is too easy to break components off the circuit board or to damage the circuit board. So don't use a lever/knife-blade on your fob. Just try to wiggle, bend, and pry the two halves apart. It is more important to break the epoxy holding the two halves together with a knife (see the next step after this one). You may even try heating the fob up with a hair dryer. I haven't tried this, but it would make the plastic a little more pliable and perhaps soften the epoxy.

Now the dangerous part!!

Slide something flat and hard between the two halves of the case. I used a pocket knife's blade. You could use a butter knife, putty knife, whatever.

Just be careful NOT to damage the circuit board. The knife will clear the components on the board as long as the board is still properly seated in the case. So push up on it before inserting the knife.

Slide the knife about half way between the two ends of the case. Then squeeze the two open ends of the case together. You're trying to pop the other end of the case open.

Don't squeeze TOO hard (like a vise or something) because you could break the circuit board. Just use your normal hand strength. If it doesn't pop, pull the knife out, and twist the bottom half (the half without the circuit board) back and forth.

Between the knife leverage, and twisting back and forth it will pop eventually. Just be patient and work at it. Don't force it, as you might harm the circuit board.

You can also run a very fine blade down between the two halves of the case. As you look down into the battery compartment, you'll see the seams between the two halves of the case. Running a blade down these cracks will help cut/loosen the sealant.