Excellent work, Anders and team at Microsoft.  This is going to revolutionalize JS much as JQuery has IMO.  Finally, we have some structure to aid in writing good JavaScript, as long as it does not take away JS' dynamic language property.

I am not sure why some folks compare this to C#... do they have any clue what TrueScript is before commenting?  This tool is awesome!

I am not sure about the other JS compilers, but as for the comparison of TrueScript to DART...well, DART's goal is to replace JS.  TS does not aim to do that; it does not have a "VM" like DART and it only compiles to JavaScript. 

I can't wait to get my hands on this.

Edit: Microosft, to make TypeScript truly awesome, please include a plug-in for VS2010.  That will definitely increase the number of people adopting the tool.