It appears that the battery is replaceable. http://www.notebookcheck.net/fileadmin/Notebooks/Lenovo/ThinkPad_X1_Tablet/P6010207.jpg
The article itself - http://www.notebookcheck.net/Lenovo-ThinkPad-X1-Tablet-Review.166563.0.html
There weren't any instructions there that I could find, but it appears you remove the 3 screws behind the stand (one is hidden behind a sticker) then somehow pry the cover off. I haven't tried it on mine yet, but the temptation is great...