There have been lots of games that immerse you in an imaginary world. This can be used in many ways to provide stimulus for writing.
A new one on the DS is The Another World. Currently it seems to be a due for release in Japan in 2009, hopefully it will come across to the UK soon after that.
Why is this one worth looking at now? Because it is being animated by Studio Ghibli, a world renowned animation studio in Japan. Click this link for some of the other things they have worked on.
I'm excited by this, watch the video and check it out.
"Ni no Kuni: The Another World follows the adventure of a 13-year-old boy whose actions lead to the death of his mother. One day, the boy encounters a fairy who gives him a book which promises to lead him to the mysterious world of Ni no Kuni, a reality parallel to his own."