We've just released JFormDesigner 5.1.1, which is now compatible with IntelliJ IDEA 12 and 12.1, has been tested with NetBeans 7.3, supports Eclipse plug-in in Java 7 VM on Mac OS X (read note below), fixes minor bugs and brings some minor improvements.
Eclipse plug-in in Java 7 VM on Mac OS X
The JFormDesigner plug-in for Eclipse depends on a JRE class that embeds Swing into SWT. Unfortunately, when migrating Apple Java 6 to Oracle Mac Java 7, the class was not migrated. There is work in progress to bring it back, but currently it is only available in JDK 7u12 Developer Preview (1.7.0_12-ea) (from jdk7.java.net/download.html), but not in Java SE 7u21 (from oracle.com). See also Java Bug 7154778 and Eclipse Bug 374199.