yeah this is how the Blackberry Keyboard did it, and of course there was the Alt key to get the punctuation marks. i think this as an optional feature would be nice, especially if the ALT key could be multi-touch enabled, you could hold the ALT key and type away for several numbers/punctuation at once with out the keyboard shifting.
I also think it is a good idea to have an option that long press will enter the Capital letter instead of symbols.
It will be agreat help for BB past users.
Thanks