Yes, it's IDEA problem.

When IDEA is executing project (in debug mode), commandline looks like:

C:\Program Files\Java\jdk1.6.0_38\bin\java" -Dvisualvm.id=4187372098827 -Didea.launcher.port=7536 "-Didea.launcher.bin.path=C:\Program Files (x86)\JetBrains\IntelliJ IDEA 12.0.1\bin" -Dfile.encoding=UTF-8 -classpath "C:\Program Files\Java\jdk1.6.0_38\jre\lib\charsets.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\deploy.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\javaws.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\jce.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\jsse.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\management-agent.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\plugin.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\resources.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\rt.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\ext\dnsns.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\ext\localedata.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\ext\sunjce_provider.jar;C:\Program Files\Java\jdk1.6.0_38\jre\lib\ext\sunmscapi.jar;C:\Users\msk\tmp\untitled\out\production\untitled;C:\Users\msk\tmp\untitled\lib\SweetHome3D-4.4.jar;C:\Program Files (x86)\JetBrains\IntelliJ IDEA 12.0.1\lib\idea_rt.jar" com.intellij.rt.execution.application.AppMain com.eteks.sweethome3d.SweetHome3DBootstrap

When I removed all that additional jars and only sh3d.jar remained, plugin was loaded successfully.

I'll try to find which library will break it, maybe some custom classloaders are used there ...

Thx and sry for mistake.