The Nokia N8 comes with a non-removable battery, at least per Nokia. Since it is a standard 1200mAh BL-4D battery, it was a mater of time someone got around to show the world how to swap it. There is already a video tutorial how to do this - all it takes is a small Torx screwdriver, and less than ten minutes of efforts.
Have a look at the video below, but bear in mind this might void your warranty. Also stay put if you are satisfied with your Nokia N8 battery life, or are not willing to carry a little screwdriver along with your spare battery everywhere you go.