The OS X wizards at The Omni Group have released an OS X based, Velocity Engine optimized screen saver. OmniJulia will only work on G4 based machines, due to its heavy reliance on the G4is Velocity Engine. According to The Omni Group:
OmniJulia is a Mac OS X screen saver module that displays constantly animating Julia sets. OmniJulia utilizes the Velocity Engine of the G4 processor and SMP cababilities, if present.
Source code is available for OmniJulia. This may serve as a useful introduction to Altivec programming, and screen saver programming (although details will change in Mac OS X final), including changing video resolutions in your screen saver.
The heart of the screen saver is some heavily commented assembly code, but there is a version of the core function written in C with the Altivec language extensions. I havenit tested or timed the C version in a while, but it is probably easier to read and will probably run nearly as fast.
You can find more information at The Omni Group web site.