The recent Tiger upgrade (10.4.6) is a step toward Windows on Intel-based Macs. Apple has a beta program for those intrepid enough to apply. The next OS, Leopard, will apparently run both Mac and PC.
I recently upgraded to OS 10.4.6. Tad buggy and slow at first, but it seems to learn as it goes (and speeds up?). Can't tell if I really like it or if I made a huge mistake.