There is one major problem with a full-fledged creative world: why would anyone want to play SMP when there is creative? Everyone would be in the creative world, and works in SMP would diminish.
This did give me an idea though. A creative world that acts as if it were single-player. You would be assigned a large lot, but would not be able to travel outside of it, and other players would not be able to enter it. You would not be able to see other players or do local chat, and maybe not even global chat. You would have creative mode, meaning unlimited blocks and flying. Perhaps once in awhile we could have 'showings' that allow players to enter these lots.
Just a thought.