There have been reports of problems with session serialization, specifically when starting/stopping/redeploying apps on Tomcat. The following is a stack trace reported by QA when running tests against the compat version of Component Showcase:

Sep 14, 2011 9:59:55 AM org.apache.catalina.session.StandardSession writeObject
WARNING: Cannot serialize session attribute org.icefaces.impl.push.DynamicResourceDispatcher$SessionBasedResourceDispatcher for session 8BC56C0BC0831DF53CC6B64D43C1E156
java.io.NotSerializableException: com.icesoft.faces.context.ResourceRegistryLocator$DynamicResourceDispatcherAdapter$DynamicResourceAdapter
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1509)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1474)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1392)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1150)
    at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1509)
    at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1474)
    at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1392)
    at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1150)
    at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:326)
    at java.util.HashMap.writeObject(HashMap.java:1001)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)