Hi,
Even though I've driven my Boxster quite comfortably in 40°-50° weather (Windstop on, side windows up, heater on vent/floor), I wish I had the heated seat option.
I am intrigued by several aftermarket kits, especially those using a carbon-fibre matrix rather than a traditional single heating coil. I am currently looking into several of the models offered by this company to retrofit into my car:
http://www.sportsimportsltd.com/seatheatac.html
These are actually less expensive than the Factory option, of course with these, you do the work.
Happy Motoring!...Jim'99