There are new requirements for screensavers in OS X 10.6 (AKA Snow Leopard). From the dev forums.
The new requirements are:
(1) Your screen saver **must** be built for X86-64, since ScreenSaverEngine is now a 64-bit app in Snow Leopard.
(2a) If your screen saver only targets Snow Leopard, then you must set the GC setting to "supported", and you can drop the PPC build if you want, since no one's going to run it.
(2b) If your screen saver must also support Leopard or earlier, then you must set the GC setting to "supported" for X86-64, and "unsupported" for PPC and X86. (Leopard's Screen Saver framework didn't support GC, IIRC.)
So this means you're going to have to port your screen saver to X86-64 with GC support, much like if you were making a preference pane, except that ScreenSaverEngine won't bend over for backward compatibility like System Preferences will. And file bugs on any example screen savers that are not being built for X86-64.