Thanks for your answers.

They raise the following question (not related with the original, tough).
Do you respect more a pianist that play from memory than one who plays with the score?
From my point of view, it doesnt matter. I enjoy the music, and dont worry about the issue (but i have to admit that someone sitting next to the pianist and turning pages is not very aesthetic).
Yesterday, i attended a piano recital, where Argerich played 2 pieces:
1) A Shubert's march, 4 hands
2) Prokofiev's symphony n°1, transcripted for 2 pianos
She played with the music in both cases. And the performance was awesome !!!

I don't know who she played with, but the traditional rule is that when playing chamber music or orchestral piano parts, use the score; if playing a concerto or solo piece, then no. I would try to err or the side of memorizing too much (e.g. I have experimented with friends memorizing piano trios), because when you play chamber music, the music should only be used in cases of emergencies. My page-turners often get frustrated because I never look at the music, so they feel useless. If you are playing a difficult passage, you really should have it memorized by performance time from practice; during accompaniment you ideally would look at your partner, and during solos, not have to worry about reading.
There is something terribly exciting about watching a solo performing play from memory. YES, it is a little bit dangerous but that danger adds to the thrill. I love that feeling I always get right before I start playing a concert when I look at the keys and think to myself, "How in the world do I make sense of this," and then somehow I do. Sheet music destroys the poetry, always reminding the performer isn't just making this up as they go along. This is even more true with singers, try watching someone perform lieder with music. Half the point is gone, they are just saying someone else's words. In addition, memorizing music will teach you so much, it improves your mind and your ability to retain other unrelated information.
To memorize the most easily, and most valuably, thoroughly analyze the structure of each piece, and understand fully how the themes interact. It is so much easier to remember, say, a fugue, if you think, "Oh, this is where he inverts the theme in the alto voice," rather than, "Okay, now, G#, A, D..." And start early. The younger you learn how to memorize, the easier it will become as you age. I've been memorizing since I was 5. I just finished memorizing a Mozart sonata I started on Thursday, and tomorrow it's off for some Scarlatti. Have fun!