Thank you scrypt for your answer.
I think that I can use the E18 with Galaxy S and a specific kernel.
A made a question directly to Fiio support about this. Here is they answer:
"When E18 works as a DAC for the smartphone, the phone will not provide power for the E18. That is to say, the E18 will not consume the power of the smartphone when the E18 plays a role of DAC. Moreover, if you choose the external charger function by changing the switch, the E18 can provide the power for the smartphone."
This is confirmed also on the official page: in the specification section is possible to read
"Battery life: 25 hours (amp only); 12 hours (AMP & DAC)."
So it seems that will be enough to enable the USB OTG function on the phone and the E18 can work also if the smartphone is not able to supply power via USB.
In the same mail, here is the answer by Fiio about when the E18 comes out:
"The E18 will start the trial-production in these days and firstly in China."
The official dealer of Fiio products here in Italy says me that in Europe the DAC/AMP will be available in the first days of december and will sell for 159€.