Nice AltiVec libraries

Cool document...and there's also the very well-documented headers for this (and the other parts of vecLib) at

so if a program used this header file in it's programing, what would happen to those of us with g3's? would programs using this header file still run under pre-G4's?
As a programmer, you would want to implement scalar versions as well and have some run-time flag determine whether or not the AltiVec instructions are available.