Nokia has addressed user complaints regarding the Lumia 800's battery, saying that a problem with the way "certain variants" of the 800 manage power can stop the phone using its full battery capacity.
If you own a Lumia 800, Nokia says you can enter the battery diagnostic tool by dialing ##634# then having a look at the Battery Status tab. If you're seeing less than 1000mAh capacity at full charge, your phone has the glitch.
A fix is on the way, though. Nokia says: "The planned software update in early 2012, as well as including many performance enhancements, will also include a fix that will enable the affected phones to access the total battery capacity."