Jenkins fail to start with an Exception due to JDK1.5 incompatibility :

java.lang.UnsupportedClassVersionError: Bad version number in .class file
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at winstone.classLoader.WebappClassLoader.loadClass(WebappClassLoader.java:68)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
at hudson.security.PermissionScope.<init>(PermissionScope.java:70)
at hudson.security.PermissionScope.<clinit>(PermissionScope.java:95)
at hudson.security.Permission.<init>(Permission.java:179)
at hudson.security.Permission.<clinit>(Permission.java:292)
at jenkins.model.Jenkins.<clinit>(Jenkins.java:3817)
at hudson.WebAppMain.installLogger(WebAppMain.java:256)
at hudson.WebAppMain.contextInitialized(WebAppMain.java:107)
at winstone.WebAppConfiguration.<init>(WebAppConfiguration.java:875)
at winstone.HostConfiguration.initWebApp(HostConfiguration.java:129)
at winstone.HostConfiguration.<init>(HostConfiguration.java:71)
at winstone.HostGroup.initHost(HostGroup.java:87)
at winstone.HostGroup.<init>(HostGroup.java:47)
at winstone.Launcher.<init>(Launcher.java:177)
at winstone.Launcher.main(Launcher.java:384)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at Main._main(Main.java:273)
at Main.main(Main.java:98)

It seems to come from usage of the last Guava release version that doesn't support JDK1.5 (http://code.google.com/p/guava-libraries/wiki/Release12#JDK6_APIs)

Change that seems to be reponsible from this error : https://github.com/jenkinsci/jenkins/commit/95c8707511e83af772d29b25d30343d2d58bcbad