[nathos]

For the most part, I'd say Apple's compatibility efforts have been equally impressive:

* 'Classic' support enabled Mac OS 9 apps to run on OS X for many years.

* Rosetta allowed for apps written for a completely different processor architecture to run on Intel (until 10.7, aparently).

Apple also made these moves before the bulk of 3rd-party applications were running natively on the new systems/architectures. The fact that Apple has been able to change low-level OS & processor architectures over the past 10 years is quite a feat.

[jwatzman]

And don't forget what I think is the most impressive, the transition from 68k to PowerPC. When they first did this, parts of the kernel were still compiled for 68k.

http://en.wikipedia.org/wiki/Mac_68K_emulator