Again it depends on the DRIVE (doesnt really matter whether its AC or DC) but does matter if it has communication capabilities such as Profibus, Modbus, Devicenet, serial etc.
Just about all drives can use an analog input like a potentiometer, a plc can send signals similar to a potentiometer to vary speed using analog outputs from the plc to an analog input on the drive. You would also probably need a High Speed Counter to read feedback from the motor to "know" speed.
This method is not as eloquent as the communication options but its a tried and true method that is usually fairly simple to implement. In many cases the communication addon for 1 drive is much higher than an analog card for the plc. With multiple drives the communication card addon could prove more useful.
Overall I agree with Dark Knight, if the drive and plc is capable of using Profibus (or any communication method) then that would be the best route.