Apple has released WebObjects 5 Update 3 to enable users to upgrade their Mac OS X systems to Mac OS X 10.1 and continue using WebObjects 5. After upgrading the operating system to 10.1, you must also download and install the Mac OS X 10.1 Development Tools, which are available from Apple Developer Connection, before installing WebObjects 5 Update 3.
Due to several incompatibilities between Mac OS X 10.1 and WebObjects 4.5.1, you should not upgrade your WebObjects 4.5.1 systems from Mac OS X 10.0.4 at this time. Apple is hard at work on an update to WebObjects 4.5.1 that will resolve the known incompatibilities and allow developers to upgrade their WebObjects 4.5.1 systems to Mac OS X 10.1. This update is expected to be available soon.