What the hell is this thing?

I have these caterpillars all over a bush in my front yard. They are evil and aggressive. If you brush up against them they lunge at you and regurgitate a greenish liquid that they try to deposit on whatever touches them (as shown in the pictures). I'm afraid of them. I think they might bite or sting, so I have avoided touching them. They have destroyed the bush they inhabit. Their appearance on my bush did coincide nicely with the arrival of my new Nikon Coolpix 4500 which I just purchased for $350 refurbished. One of the main reasons I chose this camera was for its excellent macro ability and the cool swivel design. So click the images to check out my evil caterpillar and let me know if you know anything about it. I'm very curious to know if it can sting or if it's just bluffing.

UPDATE: After further careful study of the photos I took, it would appear that this thing can sting the hell out of you. You may notice from the photos that the caterpillar has 3 sets of front legs and 3 sets of back legs. The three sets of front legs are different from the back ones though. They have pointy segmented portions that extend from the base of the leg and if you look closesly, you can just see what appear to be the little white tips of stingers protruding from inside these segmented legs. So it would appear that this biotch can sting you with six stingers at once, while simultaneously spitting some sort of green vomit at you. This is like some kind of Hollywood/Sci-fi caterpillar from another planet. Truly disturbing.

FINAL WORD: It would seem, after further investigation, that this caterpillar is harmless to humans despite the fierce show. Thanks to luvweasel (who also has a great blog at blogbybent.blogspot.com) for pointing out this page which identifies the creature as a species of Datana, also known as an Azalea Caterpillar. In my investigations, I also came across the following fantastic pages in case anyone out there needs to identify their own critters:
http://bugguide.net
http://www.whatsthatbug.com