Nothing to stop you from doing that now, you'll can tie a mini to the back of a Cinema Display's stand with some cable ties quite easily (I I have a FW backup drive hanging on the back of my iMac and you honestly can't see it from the front). But you won't have the option of a 21.5" model and you will either have a teensy 500GB Hd or no CD/DVD drive. Plus, at R13 K for the Cinema Display plus 8500 to 10500 for the mini you're not saving money over the iMac, which maxes out at R22 K.
Admittedly, Apple's Cinema Displays, while gorgeous, are wildly overpriced, so if one was prepared to compromise a little there, things get more interesting.
SSD is nice, no denying. Wake me up when a terabyte of it gets affordable.