Dungeon Siege III has been delayed til June 21 in the US, June 17 in EU. The reason is very simple: it's not ready yet.
“Square Enix is committed to delivering the ultimate Dungeon Siege experience,” said Square’s CEO Mike Fischer. “Although the final polish phase has demanded a bit more time than originally planned, we want to assure fans and followers that the additional effort will allow us to deliver an action-RPG worthy of its rich heritage.”
Given Obsidian's checkered history of releasing unfinished or buggy games, this is a good idea - examples include Knights of the Old Republic II: The Sith Lords (though that was honestly LucasArts' fault), Alpha Protocol and Fallout: New Vegas. It seems Square-Enix doesn't want history to repeat itself.