As of version 1.1, you can now choose to make a Super Flat world when starting a Creative world. As you would imagine, it is indeed flat, but only goes 4 dirt blocks deep before you hit bedrock. Although this pretty much eliminates the ability to make tunnels, it makes for a faster loading world.
Another cool thing with Creative is the mobs. During the day, slimes spawn frequently and regular monsters still spawn at night, but they don't attack you unless you attack them. No need to worry if a creeper wanders into your house, just call over the zombies next door and throw a party. Zombies and skeletons still burn up at sunrise.
Also, you can use Spawn Eggs in Creative mode to spawn pretty much any creature anywhere you like (provided Peaceful is off for the monsters). Different colored eggs match their respective mobs. Go play!