Cell Phone Batteries with Arduino?

I have a few 3.7 volt cell phone batteries - Lithium Ion. What sayest the hive about using these to power mobile Arduino projects?

I have seen this Youtube video on charging Lithium batteries but if it is dangerous then I'll give it a pass.

Sure, just run your portable project at 8MHz.
I use a MAX1811 to charge a 1000mAH LiPo in an RF remote control for me fencing club, powering a 8 MHz Promini from its VCC pin (bypassing the regulator). Plug in a 5V wallwart to charge it.
You can find similar LiPo charge controllers at Sparkfun.

Why does the project need to be run at 8MHz?

At low voltages, the crystal oscillator is not guaranteed to run at 16 MHz.

With a cell phone or any other Li-based battery, you need a protection circuit to prevent discharge below about 2.8 V. Otherwise it will be ruined.