IBM has developed a new framework that is to be the basis for future IDE's. They have released it as open source under the name Eclipse.
See
Eclipse Corner
While this IDE is almost exclusively written in Java, rather than use Swing, they have developed their own Widget framework called SWT. SWT is a set of Java class libraries that use native code to draw the widgets. This results in excellent L&F support as well as performance.
The downside is that SWT has to be ported for Eclipse to run on a Platform (currently it supports Win32 and Motif). We are rounding up people to help with the port to OS X. We are hoping to just map SWT to the existing Cocoa wrappers that Apple provides.
We could use some experienced Mac and Java developers.
If you are interested then please sign up for the platform-swt-dev mailing list at
Eclipse Mailing Lists
Thanks