I've been considering doing this myself for a while now (in fact I was very much planning on pecking CraigyB's head about it had he been able to make it to our meet in Jan), the idea being that I either want to fit an OEM sat nav unit, or use the same hole to tidily mount a VGA touchscreen for a Car-PC (however the question of whether this method can reliably take the daily hammer it would get has left my undecided so far)...
I know one of the other chaps on the board retrofitted a fully-loaded OEM sat nav setup into his S3; but I'm not sure whether he had to convert it to double-DIN first or not...
As far as I can work out from the looking at ETKA/ETOS and ELSA and the bit of examing my dash I did the last time I had my car in bits; it's possible - possibly even without any 'bodging' to speak of, but I'm not holding my breath for that. As with anything there are complications, and how severe they are depends on the spec of the dashboard you're starting out with...
If you wanted to fit a face-lift double-DIN centre console for instance, the parts catalogue lists a seperate version of the air-conditioning controls for vehicles with sat-nav - I don't know how critical this would be, as I've been looking into the conversion with only blower/heater controls in mind... It may be for a reason you could sidestep, but my point is just that there are hundreds of little caveats to consider (right down to switches at the top (they're a different size/shape) etc.)...
In terms of the job as a whole though, I'm 99% certain it's possible to do; but I guess anything is depending upon reserves of time and money... I don't think there are any BIG headaches in terms of fitting the console itself though, from what I can see it shares (almost?) all the same metalwork at the top side (where it joins the dash); it's just getting whatever equipment you need to have in it back-in...
I've started to collect a few bits and peices that I'm going to need to do the job (I'm planning to buy as little as I can get away with from dealers); but I've not made a start on it yet... If you've not gone ahead before I do, I'll keep you informed...
Regards,
Rob.