Wind- Powered phone charging bike rig
A quick google search yields a nice paper showing the theoretical amount of power a wind turbine can produce, with wind speed, blade length, air density, and "Power conversion coefficient:"

http://www.raeng.org.uk/publications/other/23-wind-turbine

I'll make up some quick numbers: 10 centimeter long blades, USB charge power of 1 Amp at 5 Volts = 5 Watts, perfect electrical conversion to this voltage (ha!), very good turbine design and generator, so conversion coefficient of ~0.3, air density as given in the paper (1.23 kg/m^3)

P = 5 Watts = 0.5 * 1.23 * (pi/100) * V^3 * 0.3
or roughly 1000 = 1.23 * V^3
yielding V ~ 9.3 meters/second, or ~33 km/hour, or ~21 miles/hour. Keep that up for an hour or so without slowing down, and you might charge your phone.

Ducting more air over your blades would help, particularly as you're unlikely to get efficient conversion to 5V for your charger or the 0.3 power conversion coefficient.