Those of us who have been using Xcode 4 since the early previews couldn’t help noticing IBPlugins – which allow third-party custom controls like the popular BWToolkit to be used within Interface Builder – were not working in version 4. Unfortunately, this is by design.
Members of Apple’s developer tools team confirmed Xcode 4 will not support editing nibs/xibs with IBPlugins in the new integrated Interface Builder. It’s worth noting your existing nibs/xibs will continue building under Xcode 4; it’s merely editing them that is not supported. The official work-around from Apple is to continue maintaining Xcode 3 alongside Xcode 4, editing your nibs/xibs in Interface Builder 3 when the need arises.
No explanation has been given for the dropped feature, nor is there a promise of it returning any time soon if at all.