You have to play on them dry but it's not as slick as you would think. Better to slip than break an ankle, but I don't recall ever having a problem when I played on it.
Also, some of the slips aren't just because the grass is slick. Knowing the surface won't skin you alive, it's easier to be more aggressive and try things you wouldn't on a hard court. That's part of why you see some guys on the ground.
[This message has been edited by TennAg (edited 7/7/2009 3:02p).]