Wonderbar:
Hi Aldo. Your post has given me slight pause, I admit. But facelift would probably come at Geneva in January 2013, then cars would not be available in the U.S. until summer 2013, and facelift Turbo S would probably come in summer of 2014. By then the new generation would be coming out in 2015, and a Turbo S buyer would be investing in a "older" facelift model when a new generation (substantially redesigned) was on its way.
Am I figuring this right?
Pretty much. 
The thing is: Porsche always puts facelift models on the market, sometimes maybe a little bit too fast for such a small manufacturer. Thinking of the first Mercedes E190 class (W201), this model was 11 (!) years on the market.
Why should I lie: When I ordered the Panamera Turbo S a few weeks ago, the person who told me that the facelift is due 2013, he also knows quite a lot of details about the project, asked me if I am crazy not to wait for the facelift ?
I asked him to give me some good reasons to wait and he started: Improved(but not completely new) look, improved(but not completely new) technology, 500-520 hp in the Turbo engine...Turbo S probably not available for another 6-12 months after facelift introduction..., better resale value.
Well...I just can't see why I should wait: The look will always bother me (I'm not a fan of sedans and especially big sedans), no new spectacular technology (car is already long enough on the market to have the regular quirks sorted out), 520 horses in the Turbo sound nice but are probably still slightly worse, performance-wise, than the 550 hp in the current Turbo S, Turbo S not available immediately and since I don't buy the car but lease it, resale value isn't really important.
So why would I keep myself enjoying the Panamera for another year or so ? It just didn't make sense, despite the fact that I really need this car as a fast family transport. 
Yes, the facelift will make the older Panamera look slightly outdated, true. Maybe this is a good thing ? I can still tell people who envy me "nah, thats the old model, the new one is really expensive...". 
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RC (Germany) - Rennteam Editor Porsche Panamera Turbo S (June 2012 delivery), Porsche 997 Carrera GTS Cabriolet PDK, BMW X5M, Mercedes C63 AMG Coupe PP/DP, Mini Cooper S Countryman All4