I recently bought a couple of Arduino Fio's with
Lithium batteries, because I found out that the
Fio is capable of charging the batteries. I can not find and
wonder if the protection system that prevents the battery
to go under 3 volts is also integrated, because I think it would
kill me battery if it wouldn't be there.
The fio board doesn't appear to have any over-discharge/under voltage protection, so it's up to the battery or external circuitry to provide that required and needed function. Some Li batteries have such internal over-discharge/ undervoltage protection but many do not, you have to check the datasheet for the specific battery you are using.