The source was already "open" before today, but under a non-free license. It was permissive, just requiring you to give Turbulenz Ltd. the option to publish your game on Turbulenz.com, and letting you do whatever else you wanted. Problem is, that would legally require you to use the Turbulenz.com API in your game's asset pipeline, even if your game didn't really need any of its features. So the MIT license gets around this restriction, with the added benefit of Slashdot coverage because OMG FOSS.
Slashdot Mirror
The source was already "open" before today, but under a non-free license. It was permissive, just requiring you to give Turbulenz Ltd. the option to publish your game on Turbulenz.com, and letting you do whatever else you wanted. Problem is, that would legally require you to use the Turbulenz.com API in your game's asset pipeline, even if your game didn't really need any of its features. So the MIT license gets around this restriction, with the added benefit of Slashdot coverage because OMG FOSS.