Hey, just wanted to tell you I had this problem throughout the weekend. Could not for the life of me figure out why the battery was not being recognized. I got the X over the battery icon on the status bar. Also, pushing that little button on the battery, gave me one light that was flashing rapidly. I tried all the resetting techniques that Apple.com's support pages stated.
I then called Apple Support and they tried the same exact procedures, such as resetting the open firmware and NVRAM, and removing the battery and the AC adapter, then pressing the power button for 5 seconds. Then, they had me reconnect everything once again, yet the battery was still not registering. Finally, Apple Support suggested that they send me a new battery for replacement or to go to my local Apple Store for an exchange.
I chose the latter, and received a new battery. The "Genius" said that sometimes, those batteries just lose their charge. I would suggest you contact Apple Support, especially if you are still under Warranty, otherwise, it's $129 for a new Macbook battery.