I'd never use the rumored mini and I don't think it will come to frutition - but if it did, I'd have no use for it. The current iPad is the perfect size for a tablet. Apple nailed it and should continue to develop in that space rather then developing a new sized iPad.
There is no reason they can't develop for both. They do it for their other products.
Also, if they essentially shrink it without lossing the actual ratio of screen size, no more real developing would have to be done App-wise, as most of them will be ready to go from the start.