CoffeeScript's now up to 0.1.3, which includes --interactive to launch a CoffeeScript REPL, and --run to compile and execute scripts in a single pass. Both options depend on a working Narwhal installation.
Now, add a minifying pass after compilation for deployment and you'll have created a dream platform for client-side scripting (-:
edit: another suggestion since one of your goals is terseness: you may want to alias "prototype" to "proto".
edit2: as much as I like the column for asignment (present in both JS itself and Ruby 1.9), I'm not really fan of the +:, -:, etc. combined operators, at least in a math context. &&: and ||: look fine though so it's perhaps a matter of getting used to it.
edit3: I just discover your underscore.js library. Do you plan to include it in the CoffeScript standard lib?
edit4: oh... And I hope you like the color of this brand new bikeshed I just built in front of your coffe shop ;-)
