It's called, erm, Wanda and the Colossus (ワンダと巨像) and, okay, it's not an ICO sequel, but it's done by the same team and seems to share a number of obvious features as far as I can judge from the official site and these magazine scans.
There's a movie to view, but Windows Media Player for OSX - as all too often - just doesn't want to know. I just hear the music and that too sounds ICO-ish.
Given that the Japanese release date is listed as 2005, I'll try not to hold my breath too long for a UK appearance. I wonder whether it might just be nudged into a PS3 title given the date and possible state of development.
Battling architectural monsters, hmmm...