Yes - we made it such that using a binding directly in the 'disabled' attribute does what you want. So you can write:
<button .... disabled="{{length == 0}}"> ... </button>
Note that this only works if you are using a data-binding, using 'disabled="false"' will still show the button in a disabled state. You can read some additional details in the discussion about "boolean attributes" here: http://www.dartlang.org/articles/dart-web-components/spec.html#binding-in-attributes
(credit to Siggi Cherem for the answer)