The legacy solution, before 1.6, was to use .attr and handle the returned value as a bool. The main problem is that the returned type of .attr has changed to string, and therefore the comparison with == true is broken (see http://jsfiddle.net/2vene/1/ (and switch the jquery-version)).
With 1.6 .prop was introduced, which returns a bool.
Nevertheless, I suggest to use .is(), as the returned type is intrinsically bool, like:
$('#dropUnit').is(':disabled');
$('#dropUnit').is(':enabled');
Furthermore .is() is much more natural (in terms of “natural language”) and adds more conditions than a simple attribute-comparison (eg: .is(':last'), .is(':visible'), … please see documentation on selectors).