LiquidWare has a Lithium Backpack which goes underneath an Arduino:
It needs two wires to feed the regulated 5V power + ground lines directly into the Arduino (I could have also connected them to the ICSP header). The switch on the battery board which is just visible in the lower left corner let’s you select between using and charging the Li-Ion battery.
Which is good, given that Lithium batteries can be a bit, ehm, finecky to deal with…
[youtube=http://www.youtube.com/watch?v=dOMfdH76oi0&color1=0xb1b1b1&color2=0xcfcfcf&feature=player_embedded&fs=1]