The funny thing is that most 'analogue' servos, are in fact digital these days. Years ago servos used specific IC's for control, but nowadays most are using microcontrollers like Atmel or PIC programmed to emulate them.
Back in the olden days, there suddenly appeared on the market "electronic" washing machines. They had pretty much the same innards as their predecessors, but the motor switching was done by a semiconductor device and small contacts in the control box rather than big contacts on the control box. It was cheaper to do and they could charge more for the product.
In much the same way, so-called digital servos are different in that they have a stepper motor (i.e. brushless) and the appropriate output stage. The input signal is called "digital", because it is in one of two states at any one time - on or off. Its just as analogue as it ever was because the information consists of the length of time the signal is "on". It doesn't really matter what is in between, discrete components, a hybrid chip or a PIC, all we are interested in is whether it works.
No doubt before long there will be a fully digital system, probably using a derivative of USB for connection and as a signal protocol, along with something really clever as a position sensing device, and I cannot help but wonder what the marketing people will call it, having used all of the appropriate words on the preceding stuff.