This is a common issue and I have the same problem on my 996. I got a few quotes (expen$ive), so I have also been looking into it as DIY.
There is a DIY here, but its performed with the engine removed from the car so not exactly what we're looking for but it does show the key steps:
http://www.pelicanparts.com/techarticles/Boxster_Tech/16-ENGINE-Camshaft_Swap_and_Chain_Tensioner/16-ENGINE-Camshaft_Swap_and_Chain_Tensioner.htm
From what I've read, we'd need special cam
holding tools (which are not the standard cam locking tool). This is discussed in the Pelican link above and again here:
http://986forum.com/forums/performance-technical-chat/45956-pulling-cam-covers.html
Also, many people have said that its highly likely that the cam tensioner pads will also need replacement. Thus, I worry that this is one of those jobs that can grow in scope and difficulty depending on what is found.
I've also read that a few people ended up with the covers leaking worse than before they started.
Right now, I am leaning towards having a shop do mine but the cost keeps me thinking that maybe I can DIY it and save a bundle.