lol, interesting topic...
First of all, I'm pretty sure those default signatures never mention the model. It's just "Sent from my iPhone" or "Sent from my iPad" Not "Sent from my iPad 2" or "Sent from my iPhone 4" or anything of the sort.
One of the only reasons I can think of for changing the signature to "Sent from my New iPad" is either to let all your friends know that you have the nice new iPad, thus making you feel a little better about yourself (whether it's a big feeling of accomplishment or just a little one). Or you're just taking a stab at the whole "the iPad 3 is named 'New iPad???'" joke. But that seems a little unlikely.
I'm not judging anyone here. I'm just sharing my thoughts, seeing as you're open to them.