I believe that it's a sawfly larvae and not a caterpillar. Sawfly adults are members of the Hymenoptera order, rather than Lepidoptera (butterflies and moths).

The reason I came to this conclusion is because of a photograph I found doing a quickie search for this little guy. If you look at the image I've attached, you can see three pair of real legs, and as many as 7 pair of prolegs. Sawfly larvae have that many, but caterpillars don't. Also, the head capsule and the leg structure just aren't right for a caterpillar.

The image, by the way, was labeled as the caterpillar of a Skipper (Lepidoptera), but no species. I believe that they are incorrect.

I haven't found out which sawfly, though. I'll continue to look around.

Here is a link that might be useful:

Your other post with more images

http://nature.gardenweb.com/forums/load/insect/msg0822153815896.html?1

Here is a link that might be useful: your other post