I think Pirlo's absence is a huge blow for Italy. He was very impressive against France and had some mesmorizing passes to Toni and Perrotta. However, I think with Pirlo out De Rossi has his chance to show his creative passing as he has with Roma. When Pirlo and De Rossi play together, De Rossi usually covers foior Pirlo and allows him to go forward but seldom does some passes of his own. Now, with him being the only creative force in midfield, he has a very good chance to prove himself yet again.
As for the formation against Spain, I don't think there will be any surprises.
------Buffon--------------
----Zambro---Panucci----Chiellini----Grosso
---------Ambrosini----De Rossi-------Aquilani----
------Camo/Perrotta-------------------------------------
------------------------------Cassano-----------
------------------Toni-----
He will be forced to play Aquilani to maintain the strength in the middle while I think defending against Spain would be very stupid since they play possession football and have the abiliy to open any defense up with players like Fabregas around.
I think Italy have to take it to them and play a littl emore Offensive although I doubt Donadoni will even think about playing this formation.
----Buffon-----
--Zamb----Pan-----Chiel-----Gros----
-------Ambrosini---------
-----De Rossi-------------------
----Camo-----Perrotta------Cassano--
-----------TOni------