I've been trying to incorporate some of the changes in the TextureRange sample code from Apple into my GL implementation -- using client storage, the rectangle extension, etc. Though it all works fine on my G4 PowerBook and 10.3, some other machines crash in glTexSubImage2D. Anyone had a similar experience?

Are you sure you're retaining the client-side texture buffer? If you use client storage, but then release the texture buffer, GL will try to texture from random memory == crash.