It has 3 terminals and is presumably a smart battery.
I bought it on ebay recently for $2.74 USD and that included postage from China to Australia. It would normally retail for maybe $40 - $50.
The phone's an LG TU550 and the battery is supposed to be model LGLP-GAJM. The sticker on the back is identical-looking to the original but the battery is larger and black, not silver like the original.
Update: I've plugged it in and apart from looking ugly (it sits on the outside of the phone and forms the back of the keypad) the only obvious misbehavior is that the phone doesn't show that it's charging (animate the battery icon) when plugged into the power adapter.
One possible reason for suspicion is the price - how can they be making a profit? (If they weren't, I'd have to wonder about their motives)
Update 23 Jul 2011: Looks like this is definitely plausible, at least for Apple laptop batteries. See Threatpost article Apple Laptop Batteries Can Be Bricked, Firmware Hacked.
You can read all the firmware, make changes to the code, do whatever you want. And those code changes will survive a reinstall of the OS, so you could imagine writing malware that could hide on the chip on the battery. You'd need a vulnerability in the OS or something that the battery could then attack, though.