I've been running the extended battery for a while but decided to swap back to the regular battery on days when I'll be near a charger frequently. The regular battery cover came off fine by pulling out on the fingernail groove when I put the extended in. But while trying to remove the extended battery cover today, it wouldn't budge. Felt like the phone was going to rip in half and the damn cover still wouldn't unsnap. I even tried carefully prying with a small key in the fingernail groove...no luck.
Did a quick google search and found this video. This guy's "credit card" removal trick for the back cover worked like a charm and the extended cover unsnapped easily. Just passing it along in case anyone else was having trouble too.