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servos are controlled by a form of PWM (50 hz frequency, 10 to 20% duty cycle).
however, those rc toys don't use servos. they use dc motors driven by an HBridge and use PWM for speed control. I don't know if this is common but the one toy that I took apart used PWM at the transmitter. the receiver just controled 2 pins on the hbrige (enable and direction).
Look on the web for circuits that use the LM18200T H-Bridge driver IC
Mainly proportional speed control is just an adc reading a pot, and setting a pwm relative to the position of the pot, the resulting pulses being fed to an H-Bridge driver circuit.
You can build an H-bridge from scratch, but it depends on really how much feedback you want from it, like current control etc. I would go for using an H-Bridge chip with all the trinkets already designed in and stable, and concentrate my efforts on the pwm using a PIC.
OOpic had a motor controller thingy using one of these chips if I remember correctly, so maybe you can get some information from that source ?
I don't know what's available in your area but check out the L293D for smaller motors and L298 for larger. You set directiona hd PWM the enable pin to control the speed. For motors over 1 A, I'd build my own Hbridge with MOSFETs.
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