In older versions you could use attr. As of jQuery 1.6 you should use prop instead:
$("#target :input").prop("disabled", true);
To disable all form elements inside 'target'. See :input:
Matches all input, textarea, select and button elements.
If you only want the <input>
elements:
$("#target input").prop("disabled", true);
To disable all form, as easy as write:
jQuery 1.6+
$("#form :input").prop("disabled", true);
jQuery 1.5 and below
$("#form :input").attr('disabled','disabled');