I submitted a defect (http://www.intellij.net/tracker/idea/viewSCR?publicId=10739) about IDEA 807 not showing JFrames when using JDK 1.3.1.
I noticed that the "command line" in the Run window has changed from (in 804):
C:\jdk1.3.1_03\bin\javaw.exe -classpath C:\jdk1.3.1_03\jre\lib\rt.jar;C:\jdk1.3.1_03\lib\tools.jar;C:\My Documents\Temp\Idea\Project Dummy
to (in 807):
C:\jdk1.3.1_03\bin\java -Didea.launcher.port=7532 -Didea.path.bin=C:\IntelliJ-IDEA-807\bin -classpath C:\jdk1.3.1_03\jre\lib\rt.jar;C:\jdk1.3.1_03\lib\tools.jar;C:\My Documents\Temp\Idea\Project;C:\IntelliJ-IDEA-807\lib\idea_rt.jar;C:\IntelliJ-IDEA-807\lib\JNIWrap.jar com.intellij.rt.execution.application.AppMain Dummy
So, it would appear that IDEA has started using a different launching mechanism, whereby it's always an IDEA class which is actually passed to the JVM as the main class, and the user's main class is passed as a parameter.
This is arguably a bit dangerous, because your program (as demonstrated by my program) can execute differently in IDEA to how it would execute directly (e.g. from a batch file).
If IDEA is going to do this, then it must be guaranteed to be identical execution - even if you're doing bizarre things with classloaders, security etc. Or there must be an option to turn this off.
And, of course, this launching mechanism must work with all the target JVMs which IDEA supports...