You could make it a project for some of the kids who don't always have their hands on the robot
You could make an enclosure and order some USB ports and hook them up with either a spare DC/DC converter or a BEC from hobbyking (although they normally give out anywhere from 4.9-5.4 volts (I think 5.25 is the max a phone can handle )) or you can use a buck converter like this one
http://www.pololu.com/product/2866
Thats 5v @ 9a and I hear it's pretty true to 5v. That's the option I would take because 9a can charge 4-9 phones. It's pretty efficient too (80-95%).
For the USB ports, if you do make an enclosure (out of water/laser jetted aluminum) you could add these cool USB ports that are overly tough but look cool
http://www.usbfirewire.com/Parts/rr-211300-30.html
I am using this on a charging rig for my LiPo batteries to charge my phone
Anyways, that's a cool project if you have the resources and time