This is exactly my thought. I'm currently using a cinewhite screen with my HD20. Don't get me wrong the image looks really good, especially in low ambient light situations, but I wish I'd gone with a grey screen to improve the blacks.
Hi, I don't know if this reply is too late, but I'll answer anyway. The HD20's weak spot is its black levels, this can be partially remedied by going with darker screens such as gray. I would go with a gray screen with the HD20.