I don't know about ThinkPad 600s, but I am currently on my third battery for my Toshiba Tecra8000. Every year (just about on the dot in June) my battery dies and starts discharging extremely fast. One week it lasts two hours, the next one hour, the next 10 minutes.
I asked the support person about this, and he told me that it was a common problem. They basically expect that the battery will stop behaving after a year. Note that the battery warranty is about a year.