Update
(This will be the last time I resurrect this, I swear.

)
Went to the Syracuse Carousel Mall last night, and figured I'd bring my iPad to see if they could fix the antenna for me. I was totally honest with the guy I spoke with - I told him that I bought it used with a chip and a small crack, and under my ownership that crack on the opposite side turned into an equal sized chip as well. I told him how I don't have a case for it, and I had it in my laptop bag in the second compartment next to my laptop. I also admitted to him that after I use my laptop and iPad, the inside of that bag must be warm for a while. (My MacBook can get pretty toasty under heavy use.)
He said everything I thought. The metal around the antenna was just fine, and scratch free. There were no dents or scratches in the body, and no nicks on the screen or the little gasket around it. It made little sense that I could have caused only the antenna to crack. Even if it had dropped or been hit in to something, the metal around it would show more or just as much damage. He said it seemed like an issue with heat regarding 3G use, and it made sense that the original owner didn't provide me with a Sim Card if that were the case.
Well, he brought it in back, and apparently one of the techs had heard about the problem before. He didn't hesitate to tell me that it was covered under AppleCare (which the previous owner had extended, likely for this reason) and that instead of a repair, he would replace the entire iPad for me.

/

This is absolute proof that if you are honest and upfront, you get the most honest assistance available to you. OP, you aren't alone in this issue. If the tech at a little Syracuse store had seen this before, I wonder how many times the techs at the 5th Ave store in NYC have seen it, or otherwise.
I am now obviously putting this iPad in a case. Before I use it at all, it's getting a Zagg Invisible Shield, then going in an Apple case, then in my laptop bag. While I told him everything about the iPad I could under my ownership, I don't know what the previous owner did - but I can say that these little things can be avoided by basic care. Some form of protection is a necessary accessory for portable electronics.