Come visit us at the Hundred Acre Woods.
Have a big bucket or wading pool for fun cool and clean time.
Who will you be camped with?
My son was 4 months old for his first Pennsic (he is 4 years old now) and we had a pack and play, a Moses basket, a carry sling and a pop up shade tent for the little one. The pop-up had a blanket on the floor and fun things for him to play with ont he floor for belly time and a mobile out of reach hanging from the top for back time. Six months old is sitting up already right? so be prepared for that.
Baby wipes and sanitary lotion are a must.
Sunscreen and bug spray that are safe for your child are very important.
I suggest you buy a whole bunch of inexpensive ear plugs and offer them around to your camp mates and your neighbors, so they can't say they were not warned.
Know that you will be secure with the conditions of Pennsic. Don't sweat the small stuff: bug bites are not so bad most of the time, sunburn is not great, the first aid tent will not know what do with a baby so have all you own medical stuff.
You don't know yet if your child is allergic to things like bee stings so ask your doctor for a prescription for an Epi Jr. just in case.
Come visit us at the Hundred Acre Wood any time. We are a camp of parents and children and we are happy to host a parent and child, especially babies. We will be int he B=blocks somewhere, not sure exactly where yet. But we will post that soon as we know. There is a Hundred Acre Woods Tribe all about us.

Erissa rocks! She has great advice.

We brought our daughter when she was 3 mo. old. Something we brought that I thought was a life saver was a small, portable swing. My daughter loved it and she could stay there for quite a while, including for her nap.

My son was 3.5 months old his first Pennsic. He absolutely loved his bouncy chair, and spent much of his "free time" seated there, watching the various goings-on around camp. We kept a small spray bottle with water in it for misting him if he got too hot. Take some Pedialyte along... the clear flavorless kind seems to go down well... and offer a small bottle a couple times a day. Even though my son drank his normal amout during Pennsic, he was more than willing to pound down that Pedialyte as well.
If you are formula feeding, I had a great system worked out for warming, cleaning, and storing bottles/formula... let me know if you want/need more info.
We used a sling carrier to get around, but brought the stroller for those afternoons of walking around in circles in camp to get him to fall asleep at nap time.
Bring both very warm and very cool clothing. I swear he was in onesies by day and fleece footie pajamas at night.

My daughter didn't go to Pennsic until she was almost 2, but we did take her camping when she was an infant. I would suggest bringing some familiar blankies, stuffed animals and toys (preferably the quiet kind for the sanity of the neighbors!) so that things aren't totally scary for the kid. Also, if you will be having different sleeping arrangements (like sleeping-in rather than a crib or vice versa) maybe try it out at home so he will get used to it in a familiar setting and so it won't be completely new.

All the other advice is great... that is just my $0.02. OH- and the hardest thing of all.... don't worry! Kids are durable and won't come to harm if you take precautions (like sanitation). So just relax and enjoy this unique opportunity. Come to visit in the B-blocks too! We aren't with Hundred Acre Wood but there are several camps with kids up there with ages ranging from teens to babies.