I'm thinking of putting in after-market heated seats in my 01 2.7L.  
Anyone tried this?  I ran into a guy who said he could do it (he's got an automotive upulstory (how do you spell this?) shop.  He said he could do it for about $400USD taxes included for both seats.
He showed me what he would put in.  It's a flat piece of heating element that looks like a tan piece of cloth and is about 1 foot by 2 feet.  The wiring, switches and installation are included in the price.
The switches are 2-position, that is, they have a High and Low setting as well as off.  
If you have seat heaters, can you comment on their ability to keep you warm on a cool day with the top down?  I have seat heaters in my other car.  They really only heat my bum and lower back.  And when I drive the Boxster with the top down, I'd usually wear a jacket if it's cold and so warmth isn't the issue.  It's actually my HEAD that gets cold.  And so I'm not so sure seat heaters would help me.