XScreenSaver (and probably most other third-party) screensavers no longer work, because, for some reason, Apple decided that screensavers should be dynamically loaded into the screensaver application rather than be separate applications, and they decided to make the screensaver application be 64-bit (because of the massive numbers that screensavers deal with?) which means that all the old 32-bit screensavers can no longer be loaded.
Oooh, I'm looking forward to JWZ's no-doubt-forthcoming rant on the subject already! Though screensavers being 64-bit actually makes a lot of sense -- many of them are very compute-intensive, and of course unlike sparc32/sparc64, amd64 code is faster than ia32 even if it doesn't need the bigger registers, as there are also
more registers than in the rather miserly ia32 register file.
Peter