Okay, I'm not usually a bug person, but when I find a caterpillar that is over 3" long in my yard, I want to know what it is! So, after a little bit of research, I am 99% sure it is a tobacco hornworm. I found info on him on wikipedia: http://en.wikipedia.org/wiki/Tobacco_hornworm
Now, what's interesting is there are no tomatoe or tabacco fields near us. No crops of any kind, really. There are fields yes, but it's all cows and livestock. So, I'm wondering how this little guy wondered upon our doorstep.
Apparently, the moth that it becomes has a wingspan of 4-5". YIKES. Now that is a serious moth! Like it's caterpillar form, I've never seen a moth of that size. It'd be rather unique around here, so I know if I see a moth of that scale, it will be our green friend all grown up!