I don't think they're upping the rates due to the CFL's because you're using less power, but because CFL's require more power to be transmitted than they use. CFL's have a significantly lower power factor (usually around 0.5) than incandescent bulbs (which have a unity (1) power factor). Power factor is simply a relationship between real power (ability to do work, measured in watts) and apparent power (voltage x current draw, measured in volt-amps or VA).
This means (without going too in-depth) that assuming a 0.5 pf, a CFL will require twice as much electricity as you'd think when you read the package (i.e. a 20W CFL @ 0.5 pf will consume 40 VA). Electric companies have to generate and transmit 40 VA for each CFL, but your electric meter only sees 20W of power consumption. Because of this, some electric companies charge extra for customers (especially commercial customers) with low power factors.
Here's a pretty good explanation of pf and real vs apparent power:
Power factor - Wikipedia, the free encyclopedia
Here's a link from a utility company explaining that they charge more for low pf:
Power Factor FAQ
