A quote from the webmaster tool help page
Hidden markup Make sure that your
rel="author"link is not invisible to humans using techniques like
display:noneor CSS. Broadly speaking, Google won’t display any information that cannot be viewed by humans.
As if there is a way for a human to see the relationship info without viewing the source HTML. It is as if google needs more incoming links into the g+ profile pages to promote them in search results….
But even google understands how stupid this rule can be in practice and allows authorship info to be specified in link tags in the header. (which is actually exactly one of the options the HTML5 spec specifically specifies)