Father Z has an English translation of the letter Benedict XVI has sent out to all the bishops regarding the FSSPX and the Williamson affair. Father Z makes pertinent observations throughout in his usual style and I don't feel the need to reproduce the letter here or pull out any excerpts.
My observation after reading it is pretty simple. If you want the letter's technical merits, you'll need to look elsewhere. I found the letter to be direct and to the point, not at all like so many other letters we get out of Rome. Benedict XVI was obviously not expecting and ended up being personally affected by the Williamson affair and it shows in his letter. While I am not one to prognosticate, I would venture a guess that this letter may prove to be a turning point, not in the FSSPX's relations with Rome perhaps, but rather in Benedict's mindset so far.