I overheard this in conversation at a Gamestop today. No joke.
It was a transformation of the phrase "That I am," a phrase which although not heard too much these days, isn't that uncommon, either.
I can't believe I didn't take the opportunity to ask him why he used "is" there. It felt like something straight out of I Can Has Cheezburger, and I can't help but think it was a reference to a meme.
It's fascinating how the internet memes are helping to evolve our everyday English. Yes, "That I is" sounds terrible to today's English speakers - but what about the coming generation? Will they chuckle the first few times they hear it, and then gradually get accustomed to the meme talk of their interwebs?
See what I did thar?