According to MacOSRumors, preliminary AltiVec acceleration in MacOS X is very promising with reports of an average of 30% performance increase.
While that sounds great, they also claim that boot time is sped up as well as launch times... which seem like odd claims, as the bottleneck on those processes tend to be hard drive access, and AltiVec's presence alone wouldn't be expected to improve this.
For a dated, but still good intro to AltiVec, check out Mackido.
