Wrong classloader is used when looking up key class – it uses cache's classloader (in JBossAS7 case this is org.jboss.as.clustering' module CL).
e.g. user could have custom key and its Transformer as part of his deployment, but only deployment's CL sees this two classes

18:04:02,866 ERROR [org.infinispan.query.backend.KeyTransformationHandler] (RMI TCP Connection(3)-127.0.0.1) ISPN014001: Could not locate key class com.google.appengine.api.datastore.Key: java.lang.ClassNotFoundException: com.google.appengine.api.datastore.Key
at java.net.URLClassLoader$1.run(URLClassLoader.java:202) [:1.6.0_26]
at java.security.AccessController.doPrivileged(Native Method) [:1.6.0_26]
at java.net.URLClassLoader.findClass(URLClassLoader.java:190) [:1.6.0_26]
at java.lang.ClassLoader.loadClass(ClassLoader.java:306) [:1.6.0_26]
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301) [:1.6.0_26]
at java.lang.ClassLoader.loadClass(ClassLoader.java:247) [:1.6.0_26]
at java.lang.Class.forName0(Native Method) [:1.6.0_26]
at java.lang.Class.forName(Class.java:247) [:1.6.0_26]
at org.infinispan.util.Util.loadClassStrict(Util.java:126)
at org.infinispan.query.backend.KeyTransformationHandler.stringToKey(KeyTransformationHandler.java:90)
at org.infinispan.query.impl.EntityLoader.load(EntityLoader.java:47)
at org.infinispan.query.impl.EntityLoader.load(EntityLoader.java:55)
at org.infinispan.query.impl.CacheQueryImpl.list(CacheQueryImpl.java:147)
at org.jboss.capedwarf.datastore.query.PreparedQueryImpl.asQueryResultList(PreparedQueryImpl.java:30)