I had good results from this site. They just identified this large caterpillar for me the other day. I got a response the very next day. My guess is some kind of Hawk Moth.
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This guy was eating my Vitex Tree.
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Here is a link that might be useful: Ask, What's that bug

That looks like the tomato fruitworm to me. I heard they like peppers also. I've killed several of them after finding holes in my tomatoes.

Below is a thread I made on them which has a picture of one. Some others I've found look more similar to your pic.

http://forums.gardenweb.com/forums/load/tomato/msg0710581316371.html

While it is indeed a Hornworm, it is not a Tomatoe Hornworm. This caterpillar turned out to be Manduca rustica or a Rustic Shinx Moth. There are over 65 species of hornworm so there are several that look similar. This one feeds primarily on Desert Willow but, will eat several other host plants including Vitex.

I am pretty sure it is a yellow-striped armyworm (Spodoptera ornithogalli)

I found one eating all my peppers.