Dear propaul:
Here is a fairly balanced article on cyberknife from the Washington Post. I looked into this treatment when I was considering options but felt that, despite its' promise, there just wasn't enough data on cure rates or SE's over the long term.
http://www.washingtonpost.com/wp-dyn/content/article/2008/11/27/AR2008112702186.html
Re salage surgery after radiation there are only a very few specialists that will try it. There is not much data that I can find on the success rate but, anecdotally, there are lots of disaster stories from those who have tried it. Divo, who is one of our forum members, posts about this from time to time...her's husband's experience was NOT good. You may want to check out her posts.
I think James C. did an excellent job of describing why the success rate might not be high.
From my personal standpoint, if my brachytherapy was to fail, under no circumstances would I choose salvage surgery. There are other options...
Hope this helps,
Tudpock