I have changed my fluid once about 40K ago on my 996, it is a straight forward task provided you are prepared and have everything you need befor hand.
There are several places to obtain LT71141 here is a list.
Your Local VW dealer under Pn.G052162A2 you will need approx 4-5 Qts , I would buy extra's. Its about 15-16 a Qt.
I ordered a giant 20 Ltr container of it since I track my car and our BMW uses the same stuff. $199.00 from Suncoast motorsports. Pn996.107.243.40
www.e-partssales.com ask for Rick.
The LT71141 is an Esso product, which Porsche has started shipping Pentosin ATF 1 Pn. 000.043.207.00 I guess this is a sutable sub of similar chemical makeup.
Here is a good diy :
http://www.renntech.org/forums/index.php?showtopic=6203
you might have to register first to see it.
or
http://www.taligentx.com/passat/maintenance/atfchange/
There are tons of DIY meterial out there on how to do this service.
Dennis