Note for Mac users: recent versions of Java for the Mac plateform occasionaly make the game to crash. Don't be scared by the glitching screen, it won't have any consequence on your computer. We are going to investigate for a solution for the following days.
Windows plateform owners don't have any problem because Java is embedded in the package they download. When there are troubles on Windows, they always come from outdated graphic drivers. Just update them to make the game run correctly and fast.
Update March 17, 2011: An fix has been found! We removed the loading screen code that was here for a long time and it seems to work again!
Java for Mac troubles