Over 10 years ago, during the transition from Mac OS 9 to Mac OS X (Rhapsody), Apple promised write-once, deploy everywhere functionality ("Yellow Box") to developers in the then upcoming Mac OS X platform.
Yellow Boxes for Intel and Mac OS would in theory make the Yellow Box the premiere choice for cross-platform development, because developers could deliver applications that run on Rhapsody (both PowerPC and Intel), Mac OS, Windows NT, and Windows 95 - all using tools derived from NeXT's highly regarded, object-oriented development environment. According to Apple, an application written for the Yellow Box can simply be recompiled for a different platform, or even shipped as a single, large file containing executable code for multiple platforms. -- May 26, 1997
By 2000, however, it was clear that the Yellow Box was dead.
OK.. so we're really flogging a dead horse here. But the fact is that Yellow Box/NT licenses were promised (contracts in hand), and yanked away. Steve Jobs' recent responses to these questions seem to be "Why not sell your product on the Mac?".
The story laid dormant until about 2 years ago when a questionable Page 2 post revived rumors of Yellow Box (full text of post):
Jobs is well aware of the risk and, as soon as he decided to revive the dormant OS X-on-Intel 'Marklar' project, launched a parallel project (now known internally as 'Dharma') of reviving (here's the big thing)... The Yellow Box for Windows. As you probably know it, the Yellow Box for Windows was NeXT's project of porting Project Builder (known as Xcode today) and the complete NeXT API (known as Cocoa today) to Windows, allowing developers to create a Windows binary by simply ticking a check box.
What would Apple use this development environment for? The poster claimed that Apple had ported Safari to Windows:
As an example of the power of the Dharma project, Apple has ported Safari to Windows and an internal build of Apple's browser (2.0.2, v.417.108) actually runs on Windows (XP required), complete with Quartz anti-aliasing. It is reported to be fairly stable, even if the Java and Flash plugins still aren't working, due to their dependency to third-party code. Apple plan to release the Windows version of its browser for free. In fact, this one was easy to do since they had to port WebKit in order for the Cocoa framework to be complete.