A developer known as Businesscat2000 has succeeded in porting iOS apps to run on the PlayBook OS.
On the CrackBerry Forums he explains how the accomplishment works:
The CPU isn’t emulated on Playbook (though it is on Windows). It works very similarly to how WINE works to run Windows applications on Linux. The app binary is mapped into memory and imports are resolved to point to my own implementation of the various APIs needed. iOS actually uses a few open APIs already, which Playbook supports just as well (GL ES, and OpenAL). The bulk of the work has been in implementing all of the objective C classes that are required. The ARM code of the applications run as-is – the armv6/v7 support on PB/iDevices are pretty much identical, and the code is designed to run in USR mode. No SWIs, GPIO accesses or any of that kind of shenanigans.
This isn’t likely to result in a way for average PlayBook owners to run iOS apps; however, it is an impressive feat.