Currently, 2.8 does not run with JBoss 5.x because 2.8 removed some JMX classes (see below).
java.lang.NoSuchMethodError: org.jgroups.jmx.JmxConfigurator.registerChannel(Lorg/jgroups/JChannel;Ljavax/management/MBeanServer;Ljava/lang/String;Ljava/lang/String;Z)Lorg/jgroups/jmx/JChannel;
at org.jboss.ha.framework.server.JChannelFactory.registerChannel(JChannelFactory.java:1019)
at org.jboss.ha.framework.server.JChannelFactory.createMultiplexerChannel(JChannelFactory.java:279)
at org.jboss.ha.framework.server.JChannelFactory.createMultiplexerChannel(JChannelFactory.java:309)
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:597)
at com.sun.jmx.mbeanserver.StandardMBeanIntrospector.invokeM2(StandardMBeanIntrospector.java:93)
at com.sun.jmx.mbeanserver.StandardMBeanIntrospector.invokeM2(StandardMBeanIntrospector.java:27)
at com.sun.jmx.mbeanserver.MBeanIntrospector.invokeM(MBeanIntrospector.java:208)
at com.sun.jmx.mbeanserver.PerInterface.invoke(PerInterface.java:120)
at com.sun.jmx.mbeanserver.MBeanSupport.invoke(MBeanSupport.java:262)
at javax.management.StandardMBean.invoke(StandardMBean.java:391)
at org.jboss.mx.server.RawDynamicInvoker.invoke(RawDynamicInvoker.java:164)
at org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:668)
at org.jboss.messaging.core.impl.jchannelfactory.MultiplexerChannelFactory.createControlChannel(MultiplexerChannelFactory.java:130)
at org.jboss.messaging.core.impl.postoffice.GroupMember.start(GroupMember.java:116)
at org.jboss.messaging.core.impl.postoffice.MessagingPostOffice.start(MessagingPostOffice.java:367)
at org.jboss.messaging.core.jmx.MessagingPostOfficeService.startService(MessagingPostOfficeService.java:462)
at org.jboss.system.ServiceMBeanSupport.jbossInternalStart(ServiceMBeanSupport.java:376)
at org.jboss.system.ServiceMBeanSupport.start(ServiceMBeanSupport.java:269)

and

2009-05-06 12:23:15,553 ERROR [org.jboss.mx.server.registry.BasicMBeanRegistry] (main) Cannot register MBean
java.lang.NoClassDefFoundError: org/jgroups/jmx/JChannelFactoryMBean
at java.lang.Class.getDeclaredMethods0(Native Method)
at java.lang.Class.privateGetDeclaredMethods(Class.java:2427)
at java.lang.Class.privateGetPublicMethods(Class.java:2547)
at java.lang.Class.privateGetPublicMethods(Class.java:2557)
at java.lang.Class.getMethods(Class.java:1410)
at org.jboss.mx.metadata.StandardMetaData.build(StandardMetaData.java:208)
at org.jboss.mx.modelmbean.XMBean.<init>(XMBean.java:224)
at org.jboss.mx.server.registry.BasicMBeanRegistry.registerMBean(BasicMBeanRegistry.java:202)
at sun.reflect.GeneratedMethodAccessor98.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.jboss.mx.interceptor.ReflectedDispatcher.invoke(ReflectedDispatcher.java:157)
at org.jboss.mx.server.Invocation.dispatch(Invocation.java:96)
at org.jboss.mx.interceptor.AbstractInterceptor.invoke(AbstractInterceptor.java:138)
at org.jboss.mx.server.Invocation.invoke(Invocation.java:90)
at org.jboss.mx.interceptor.ModelMBeanOperationInterceptor.invoke(ModelMBeanOperationInterceptor.java:140)
at org.jboss.mx.server.Invocation.invoke(Invocation.java:90)
at org.jboss.mx.server.AbstractMBeanInvoker.invoke(AbstractMBeanInvoker.java:264)
at org.jboss.mx.server.MBeanServerImpl.invoke(MBeanServerImpl.java:668)
at org.jboss.mx.server.MBeanServerImpl$3.run(MBeanServerImpl.java:1431)
at java.security.AccessController.doPrivileged(Native Method)
at org.jboss.mx.server.MBeanServerImpl.registerMBean(MBeanServerImpl.java:1426)
at org.jboss.mx.server.MBeanServerImpl.registerMBean(MBeanServerImpl.java:376)
at org.jboss.ha.cachemanager.CacheManager.registerCache(CacheManager.java:266)
at org.jboss.cache.CacheManagerImpl.getCache(CacheManagerImpl.java:124)
at org.jboss.ha.cachemanager.CacheManager.getCache(CacheManager.java:244)
at org.jboss.ha.framework.server.HAPartitionCacheHandlerImpl.acquireCache(HAPartitionCacheHandlerImpl.java:53)
at org.jboss.ha.framework.server.ClusterPartition.startService(ClusterPartition.java:355)
at org.jboss.system.ServiceMBeanSupport.jbossInternalStart(ServiceMBeanSupport.java:376)
at org.jboss.system.ServiceMBeanSupport.pojoStart(ServiceMBeanSupport.java:216)
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:597)