Fast, Cheap, and Good - Choose any Two
JavaScript - 184 169 (with jQuery)



b="input",a="&lt;input type=checkbox&gt;",c=":checked";$("body").html("SELECT ANY TWO"+a+"FAST"+a+"GOOD"+a+"CHEAP").click(function(){$(b).not(c).attr("disabled",!!$(b+c)[1])})


http://jsfiddle.net/L33JK/16/

EDIT: improved with help from @Daniel Lisik - https://codegolf.stackexchange.com/a/26805/16278