Request clarification: Does readonly apply to checkboxes and radio buttons?

Although "readonly" could be useful for checkboxes and radio buttons, 
the HTML 4.0 DTD has the comment:

 readonly    (readonly)     #IMPLIED  -- for text and passwd --

Some developers have taken this as a normative statement that "readonly" 
must be ignored for other control types. DOM2 interpreted this comment 
as an explicit restriction:

    readOnly of type boolean
    This control is read-only. Relevant only when type has the value "text"
    or "password". See the readonly attribute definition in HTML 4.01.

If the comment was not intended as a restriction, a clarification is 
necessary, since it is being cited to limit functionality of readonly in 
Web browsers and in related W3C standards.