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FYI There's now support for marking up your logo using 
Webmaster level: all. Today, we're launching support for the markup for organization logos, a way to connect your site with an iconic image. We want you to be able to specify which image we use as your logo in Google search results. Using Organization markup, ...
Peter Driessen's profile photoScott Hendison's profile photoJohn Mueller's profile photoGeorge Phillip's profile photo
One thing to keep in mind is that the marked up image is not used in the same way that authorship is shown in search results, but rather it may be used when our algorithms choose to show information about your business / website in the Knowledge Graph area next to the search results. 
Hey +John Mueller appreciate the clarification there.  Was getting quite excited.

For a number of sites, I can already see a company logo in that knowledge graph area - any point implementing this schema if that's the case? Also, do you have any idea on appropriate dimension sizes - and how "creative" can the logo be (ie does it have to be the logo, or can it be logo + tagline, logo + call to action)
Thanks for this addition, and I hoped it would solve the problem for a handful of local biz clients that each have images showing that pull from their Google+ local page.   I've tried reordering photos a few times to get the "right one" to appear, usually with no luck so I've been looking forward to this...

Nothings changed for a name search yet, but it's only been a day since i added the new logo markup - However, these pages also each have a rel=publisher tag going back to their Google+ page...  Might that be a problem, or would the logo markup take priority?  
These kinds of markup changes can take a bit of time to get picked up, and for the algorithms to determine that they're relevant for the Knowledge Graph - I'd definitely give it a few weeks, and even then, as with other kinds of markup, it's not guaranteed that we'll show it every time. As we understand how webmasters use it, I'm sure its use will evolve over time as well :)
I tried adding this for a large Australian company but to no avail. I even used the right code (i have replaced company info with xx)

<div itemscope itemtype="" style="display:none;">
    <a itemprop="url" href="">xxx</a>
    <img itemprop="logo" src="/xxx"/>
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