Blacklist creation error on boot - Google App Engine for Java | Google Groups

As documented elsewhere, if you see this:

Jan 26, 2010 5:09:32 AM com.google.appengine.tools.development.agent.impl.BlackList initBlackList
SEVERE: Unable to read a jre library while constructing the blacklist. Security restrictions may not be entirely emulated. /System/Library/Frameworks/JavaVM.framework/Versions/1.6.0/Home/lib/plugin.jar

...then do this

cd /System/Library/Frameworks/JavaVM.framework/Resources/Deploy.bundle/Contents/Home/lib
sudo ln -s plugin.jar plugin2.jar

and all will be well.