leather != green
I certainly hope that leather is not an option! (So far, it isn't.)
Some people have installed leather onto their 2001-2003 Prius aftermarket. Ask your dealer if they have a preferred shop, or see if ClassicSoftTrim has a shop near you. Be warned if you have side air bags - at least in the 2002-2003 Prius they deployed from the side of the seat, so you have to be careful that the leather installation does not interfere with the side air bags' deployment in case of an accident. (Special break-away threads must be used.)
I've had a car with leather seats once. They were a bear to upkeep (cleaning, softening, keeping the cracks out). They were hot in the summer (even through clothing), and cold in the winter. Myself and anything placed on the seats didn't stay there long - slid all over the place. I'd much prefer cloth seats, like the Prius comes with, over leather seats any day, and that's before the environmental and ethical reasons!
OK, some people might be past the whole idea that an animal (usually a cow) had to die to provide you with the leather seats that you want.
However - what about the environmental impact of raising that cow? Trees had to be cleared, land that could've been used for food crops was used for cattle-feed. Pesticide use for the food, and general farm run-off polluting water supplies. Probably more petroleum usage raising/feeding/transporting the cattle than if you got vinyl (plastic) seats!
The modern tanning industry is not known for being environmentally-friendly either. Heavy chemical usage normally (makes the leather non-biodegradable), and more waste-water runoff (full of nasty chemicals) from the tanning factories. Communities around many tanning plants report higher cancer rates.
So you take a ~50MPG car (low CO2 emissions), with AT-PZEV/SULEV emissions (extremely low smog-producing emissions), and install leather (very non-environmentally friendly product)? Just doesn't make sense to me.