That syntax has been changed, actually. Now, to allocated an owned pointer allocated on the heap, you use the Box type:
> let a = Box::new(2);
I believe it's an unstable feature currently to use the "box" keyword to heap-allocate a value, which can also be used in pattern matching. I saw somewhere that "box" will be made into an overloadable operator, such that you can easily create reference-counted and atomically reference-counted types.