Coming from a Chinese Blizzard Fansite Summit, news is that the upcoming StarCraft II
beta will include a map editor
for you guys to toil around with. The beta will start out in North America and then head to other countries and will only be multiplayer. I'm guessing they will either save a single player demo for later on or not release one at all. Seeing as Blizzard would want to flesh out and test their servers for when the game is released, I can see why it will only be multiplayer. You'd hate to have Blizzard's servers not be able to handle the infllux of players once the game gets its release.