Li-Po shield for Arduino with in-line recharging, soft-off and voltage protection?
You could use the Polulu push-button power switch together with the LiPower shield if you cut the track that joins the two 5V header vias then use the switch to connect them.  

As far as I know the design of these shields (with two rows of header vias) is specifically designed with this sort of modification in mind.  

I'm not sure what the power-up state of the Polulu switch is - so you may need to push the button on the board in order to turn the power on initially.