I'm surprised noone's added the obligatory "let's have a water game!" comment, yet
While it would be incredibly cool, it's also entirely impractical. I suppose you could try to hold competitions in venues with pools... but it really would be a huge hassle.
I can see it now.. "yes, I'd like to reserve the pool for a robotics competition." "The pool?" "Yes, the pool."
In all serious, though, I agree with Steve W. Carpet is the easiest material to use, and I forsee FIRST sticking with it, at least for the main flooring of the field. Other materials may be used, sortof like the HDPE and wire mesh in previous seasons, but the main flooring will likely stay carpet. It's just the easiest to deal with.