I've got (I believe) a similar issue with a front bearing. From what I've read, it's not too terribly bad IF you have the right tools (ie, the SIR wheel bearing kit), which are a bit pricey. Getting the axle nut off I guess is a challenge as well---it's supposed to be torqued to something over 300 ft-lb, IIRC.
You'll get more complete answers from those who've done it, most likely. Also, check out:
http://www.pelicanparts.com/techarticles/Boxster_Tech/40-TRANS-Wheel_Bearings/40-TRANS-Wheel_Bearings.htm