Against the Standard definition it seems InfoPath does evaluate CDATA
values.
I would expect:
<contentBlock key=''testkey''><![CDATA[ <iframe
src=''http://a-URL.foo/default.hml'' name=''testframe''></iframe>
]]</contentBlock>
in a text field to be shown as:
<iframe src=''http://a-URL.foo/default.hml'' ...