I have to say that I've thought about doing this for "forfiveone" as living in Canada its darn cold, but I was very surprised in that with the heated seats and the car heater I've never felt that cold for very long. I would say at worst you will be cold for 1 minute, but the heated seats warm up nice and quick and the heater in the car does the same.
But if you do want to get this done the European models have an option for a remote start and a temperature module that will keep the car at a certain temperature automatically.
I don't know how easy it will to get this on a North American smart as the ignition kill system might just not allow the car to start.
If you really looking for one, best contact someone in Europe that can source the part and instructions on installing if at all possible.
I would recommend Paul from WellSmart http://shop.wellsmart.co.uk
as he has a very good selection of parts and has even sourced some special items for me that weren't on his site.
Another vendor I've used is SPD, http://www.smart-parts-direct.com
but in my personal dealing with SPD I found them to not understand the new 451 very well, but after some hand holding I was able to get the support I needed from them.
I deal with a few people in Germany, but due to language barrier I wouldn't recommend trying it out for something like this as you'll probably need a lot of technical support. If you are fluent in German let me know and I'll send my contacts your way.