I’ve been working with keypads for my master thesis project and from what I know and learned I can tell you this:
Unfortunately I’ve never seen a computation format keypad that also has the alphabets. (Perhaps you can consider putting rub-on letters on this type of keypad?)
If I were you I would place ABC on the top (against the 123 at the bottom).
Just think about it…keypad skills are more “in the muscles” than in the “eyes”. This is clear when you enter your pincode in an ATM; you remember the movements you have to make with your thumb or finger and it almost gets some kind of a mechanism to you. Funny that when using your mobile phone, it is also natural to take the oposite layout for granted. I believe that when you want to make use of the computation format keypad, you would feel more comfortable with having the letter layout as you would have with a mobile phone (you’re used to this when you write an SMS!).