I've opened dozens of these battery packs as follows:
All you need is a heavy duty X-acto knife handle (#5) with a #19 blade.
Lightly cut through any labels that might be spanning the case halves.
Force the blade into all 4 corners between the case halves as far as you can. You need to use a lot of force so put the pack on a table and be careful to keep your hands clear of the blade in case the blade slips (or breaks, but this has never happened to me).
Find the corner that you were able to insert the blade the deepest into and re-insert the blade. Then twist the blade and the pack should split apart.
This procedure will slightly damage the corners of the pack but it will be hardly noticeable. About 20% of the packs won't open this way, in those cases you need to score into the ends of the packs as well. This is much harder to do, but always works if you spend enough time at it.
-Katie
Edited: 25 Apr 2010, 2:00 p.m.