The main advantage of the z-buffer in my opinion is that you can stream geometry through it so companies like Pixar can make images of HUGE models. However, in an interactive setting you don't want to do that, so games use pretty small polygon sets that can fit in main memory. The image quality in games will be better once ray tracing is interactive. When will that be?
Intel has just demonstrated a 80 core teraflop chip that looks ideal for ray tracing. According to recent tests, full ray tracing is about 20 times slower than it needs to be on a 2GHz dual-core opteron 870. If Intel's numbers are realistic (we'll have to wait to see what the meory implications are) then the slightly smaller Larabee system will be plenty fast enough for good ray tracing on a single chip.
What does this mean? I predict it means the z-buffer will soon exist only in software renderers at studios, so if you are an undergraduate don't bother learning OpenGL unless you need to use it in a project or job soon. I doubt OpenGL will die entirely due to lagacy issues, but it is not something to learn unless you have to. I speak as a person that spent much of my early years writing FORTRAN code!