It seems that there are still plenty of applications where only the big boys (DB2 and Oracle) really succeed. As much as I like to apply PostgreSQL (or even MySQL) to back end solutions, there are still times when one of those insanely advanced (and usually complex) features are needed to meet your requirements. Oracle being the biggest player on the block is often the one turned to to tackle those types of problems. These installs often end up on “big” hardware, which puts the idea of Oracle on Apple in an interesting position. The new Apple server hardware seems to perform well with a more reasonable cost than the traditional big hardware players (Sun, IBM, large x86, etc).
Remember what a big splash the Virginia Tech Apple supercomputer made when it was first announced? It wouldn’t surprise me if someone is doing something similar for their database back end using Oracle on top of a whole bunch of Apple hardware.
The Virginia Tech system is still ranked #7 on the Top 500 list.