Details of the game haven't been fully disclosed, but a sequel to ICO is in the works.
Whether it's a direct sequel or a side-quest as suggested is not certain, but it's not exactly the world's best kept secret.
Rumours were first fuelled when the SCEI team put out a recruitment ad looking for programmers, whilst using a graphic involving three of the ICO horned-hat boy, on horseback, suggesting that this would be the project that the sought programmers would work on. Duh.
An official announcement is planned for December at the Annual Playstation Party held in Japan, along with announcements of exciting third party projects.
We'll have more for you then.