Zeke's original proposal, slightly modified by Darius, would look like this:
<obs conceptId="16124" answerConceptId="116" answerLabel="Patent has a hat" style="checkbox" toggle="hatColors"/>
If yes, <obs conceptId="16125" answerConceptIds="16127,16128,16129" answerLabels="red,green,blue" style="Radio"/><br/>
<obs conceptId="16126" answerConceptId="116" answerLabel="Hat is warm" style="checkbox"/>
If you check/uncheck the 'Patient has a hat' box, then all input elements in the 'hatColors' div/span will be enabled/disabled.
Dave, sounds like you're envisioning something a bit different, i.e. checking/unchecking the box shows/hides a div? Could you give a pseudo-html-code fragment of how you'd like to be able to write a form?
(Also, conditionally showing/hiding content based on pre-existing info, e.g. is the patient male or female? were they enrolled in the diabetes program? can be done with the new includeIf and excludeIf tags.)