Absolutely will not happen because it will expand the size of the box too much.
Also, the lack of keyboard/mouse/monitor is what forces a lot of upsale to the iMac 21". If you leave the monitor as the only missing component you would definately see a rise in mini sales at the expense of the iMacs (customers see savings over performance difference). As a former mac salesman, I found it quite easy to upsell from a mini to iMac (unless their needs specifically made more sense for the mini).