Here are my concepts. I decided to use the Quad Bot from Tears of Steel since I think it lends itself to manipulation and interpretation than Suzanne (and it works really well with the captcha). Though it's not quite as obvious symbol of Blender as Suzanne, I personally at least see it used quite a bit in the Blender community and immediately recognize it as closely accosted with Blender.
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Inspired by gandalf3's suggestion of using missing image textures.
I do like David's broken blender Idea a lot too. If someone can pull off a good render of it I'd go for that just as well.
I accidentally rendered this one at low-res, low samples. I'll fix that sometime.
The posing on this one could use a little work.
(I'm not too swift with armatures so if someone could give me a little guidance on how to move the feet on his rig that would be cool.)
I know the rendered Quad Bot doesn't technically fit with the minimalist flat look of the main site UI. But I think it actually looks pretty good as long as the backdrop matches the site background color, it looks like he's sitting there on the page staring at you.
The Quad Bot is licensed (CC) Blender Foundation | mango.blender.org.