For some reason fullscreen does not work very well on Mac's, at least it doesn't work for you and me.
Your computer shouldn't be at 100% CPU usage. I have a 2.53GHz processor and I've never had any problems between it and Blockland. Are there other applications running in the background?
Does you laptop overheat? That would cause it to shutdown.
Actually it's a V20/Lion issue. When V19 was out, it didn't happen. Also, I turned all my mods off (Not deleted, just turned off) Now its at about 50% usage (I also turned render distance to lower)
Laptops can overheat, and not shutdown. But if it got to the point of danger, the computer would do so in self preservation. Thats never happened to me, I just prefer to keep it cooler.