= String.fromCharCode(this.index + 65);
65 is the ASCII code for "A", so you're just incrementing the letter using the current index.
I had to tweak it a bit to get it to do exactly what I wanted, but this is what I came up with and it works great.
this.Cell2.rawValue = String.fromCharCode(this.parent.index + 65)+".";
I put the script under "calculate" for the table cell and it numbers the first and all consecutive rows as A., B., C., D.,and so on.
Thanks for you help!