This appears to be an issue with the JVM under Leopard (Mac OS X 10.5), but I started getting the following error within Eclipse when using the ResourceBundleEditor plugin:
It also apparently happens with other plugins/apps and is an issue with the AWT kit in the new JVM. After pouring through all of my Eclipse config files and determining that I definitely was NOT specifying StartOnFirstThread anywhere, I found this fix:
Within the Eclipse application bundle, in the file Contents/MacOS/eclipse.ini, add this line:
Save the file, and restart Eclipse. Problem solved.