Sinobug

This is a family Lasiocampidae moth caterpillar. Other than that I am unaware of specifically what genus they belong to. They can grow to truly impressive sizes. In addition they can remain completely hidden or if disturbed put on a defensive display....
http://www.flickr.com/photos/itchydogima...
http://www.flickr.com/photos/itchydogima...
http://www.flickr.com/photos/itchydogima...

@Mishall
And to everyone.....
DO NOT underestimate the capacity of these caterpillars to inflict intense pain. I have only slightly brushed against some species and received a most painful and prolonged injury. On one occasion, I unknowingly stood up amongst low-hanging tree branches and brushed the back of my neck against the underside of a leaf bearing an entire hatching of Limacodids. The intense burning lasted for more than 24 hours. I have also been stung while photographing individuals while trying to set up shots and inadvertently coming into contact with them. Beauty = Danger.

@Ashish Nimkar
Thanks for your input Ashish. I think it will be hard to confirm ID from pictures alone however.

@Chief RedEarth
Call it a helipad, a target, a bullseye......but I think the caterpillar intended it to be a fake eye, large enough to belong to a snake or a bird or at least belonging to a creature larger than the caterpillar is, as a deterrent against potential predators.

Definitely looks like a Crambid moth (family Crambidae) and based on the visible pattern and your location, I would reckon this is Pygospila tyres.
http://www.flickr.com/photos/itchydogima...

The "bell" shape of these moths is typical of family Tortricidae, the tortrix moths.

@bayucca
Certainly similar.
http://www.flickr.com/photos/itchydogima...

This is no coincidence. I have spotted this species on the same type of leaf environment on several occasions.....
http://www.flickr.com/photos/itchydogima...
http://www.flickr.com/photos/itchydogima...
http://www.flickr.com/photos/itchydogima...

Thank you all for your comments. And that Darwin quote courtesy of John La Salle is very appropriate and profound.
As an update, I revisited the site this spotting yesterday, 5 days since taking the picture, and the poor caterpillar was in the same place, still alive and still burdened.
It has been demonstrated that the presence of the wasp larvae somehow manipulate the "brains" of their various victim hosts into 'playing safe' once again to prevent the caterpillar coming to grief before the wasp life cycle has been completed and all would be lost.

Yes indeed. The adult had not made it to a tree trunk or higher ground, but had emerged from its final skin in the leaf litter next to a running creek.