Looks like you will be able to vote for your favourite artists that have played the festival for next years event.
From their website:
The tenth edition of Primavera Sound is gathering momentum, and this year we have decided to take the usual participation of the festival’s public through our forum, one step further. To this end we have created a questionnaire in which all the registered users of our forum can vote for their favourite concert in the history of Primavera Sound. We have chosen a limited number from each year.
To participate in the survey all you have to do is be registered on the forum of our web (http://www.primaverasound.com/foros/) and fill out the questionnaire which is at the following address http://encuesta.primaverasound.com.
The process is simple: Choose the years you have been to the festival, and once you have done this chose five artists from each edition, except in 2001 in which there are only two options. From this first stage we will get 50 artists, from which 25 will be chosen in the second stage. From these 25 acts we will ask the public to chose who should play at Primavera Sound 2010.
We will do everything possible to bring the artists who are chosen in this survey to our tenth anniversary, we think that it is a nice way to celebrate ten years of something which has become for many people (including ourselves) an unmissable musical event.