After running ant install.demo (which takes fooooooreeeeevvvvveerrrrrrr !) I try to run a simple 'ant start.demo' and get first a pop-up from eclipse:

JVM terminated. Exit code=13
/usr/bin/java
-Xms40m
-Xmx384m
-XX:MaxPermSize=256m
-jar /opt/Download/JBPM/jbpm5/jbpm-installer/eclipse//plugins/org.eclipse.equinox.launcher_1.1.0.v20100507.jar
-os linux
-ws gtk
-arch x86
-showsplash
-launcher /opt/Download/JBPM/jbpm5/jbpm-installer/eclipse/eclipse
-name Eclipse
--launcher.library /opt/Download/JBPM/jbpm5/jbpm-installer/eclipse//plugins/org.eclipse.equinox.launcher.gtk.linux.x86_1.1.1.R36x_v20100810/eclipse_1309.so
-startup /opt/Download/JBPM/jbpm5/jbpm-installer/eclipse//plugins/org.eclipse.equinox.launcher_1.1.0.v20100507.jar
-exitdata 8c0005
-data .//workspace
-plugincustomization ./eclipse.preferences.ini
-vm /usr/bin/java
-vmargs
-Xms40m
-Xmx384m
-XX:MaxPermSize=256m
-jar /opt/Download/JBPM/jbpm5/jbpm-installer/eclipse//plugins/org.eclipse.equinox.launcher_1.1.0.v20100507.jar

Then at the start.humantask I get:

start.human.task:
[java] Failed to instantiate SLF4J LoggerFactory
[java] Reported exception:
[java] java.lang.NoClassDefFoundError: org.slf4j.impl.StaticLoggerBinder
[java] at org.slf4j.LoggerFactory.bind(LoggerFactory.java:121)
[java] at org.slf4j.LoggerFactory.performInitialization(LoggerFactory.java:111)
[java] at org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:268)
[java] at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:241)
[java] at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:254)
[java] at org.hibernate.cfg.Configuration.<clinit>(Configuration.java:151)
[java] at java.lang.Class.initializeClass(libgcj.so.10)
[java] at java.lang.Class.initializeClass(libgcj.so.10)
[java] at org.hibernate.ejb.Ejb3Configuration.<clinit>(Ejb3Configuration.java:107)
[java] at java.lang.Class.initializeClass(libgcj.so.10)
[java] at org.hibernate.ejb.HibernatePersistence.createEntityManagerFactory(HibernatePersistence.java:124)
[java] at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:52)
[java] at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:34)
[java] at org.jbpm.DemoTaskService.main(Unknown Source)
[java] Exception in thread "main" java.lang.NoClassDefFoundError: org.slf4j.impl.StaticLoggerBinder
[java] at org.slf4j.LoggerFactory.bind(LoggerFactory.java:121)
[java] at org.slf4j.LoggerFactory.performInitialization(LoggerFactory.java:111)
[java] at org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:268)
[java] at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:241)
[java] at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:254)
[java] at org.hibernate.cfg.Configuration.<clinit>(Configuration.java:151)
[java] at java.lang.Class.initializeClass(libgcj.so.10)
[java] at java.lang.Class.initializeClass(libgcj.so.10)
[java] at org.hibernate.ejb.Ejb3Configuration.<clinit>(Ejb3Configuration.java:107)
[java] at java.lang.Class.initializeClass(libgcj.so.10)
[java] at org.hibernate.ejb.HibernatePersistence.createEntityManagerFactory(HibernatePersistence.java:124)
[java] at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:52)
[java] at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:34)
[java] at org.jbpm.DemoTaskService.main(Unknown Source)
[java] Java Result: 1

start.demo:

BUILD SUCCESSFUL
Total time: 30 seconds

This is all done with the latest build (yesterday, 20 Oct 2010) installer from hudson.