According to an official statement published on Nokia’s support forums (via WMPoweruser), some early Lumia 800 models have a firmware issue that negatively affects battery life by rendering the device unable “to access the full battery capacity”. However, since this is a software problem, it can be easily fixed with an update, which Nokia has promised for early 2012.
If you don’t want to wait for the update, you can even contact Nokia’s support for a replacement device. Nokia also provided instructions on how to check whether your phone is affected by this issue:
Anyone who wants to know if their phone is affected can run the battery status test from the diagnostics tool already installed on their phone. (The tool is designed for service use and only displays approximate values.) Dialling ##634# opens the diagnostics tool. By accepting the disclaimer and then selecting ‘Battery Status’ from the list of options, anyone will be able to see their available battery charge capacity. If your full charge capacity reads less than 1000 mAh then your phone is affected with this specific issue.