The eleventh IPCO conference took place June 8-10, 2005, at TU Berlin.
The proceedings have appeared as vol. 3509 of Springer's Lecture Notes in Computer Science (LNCS).
For the history of the former IPCO conferences, see Gerhard Woeginger's IPCO page.
In conjunction with IPCO XI, there was a summer school on June 6-7, 2005, also at TU Berlin.
The call for paper is available as ASCII text here.