Using cassandra 1.0.7, I got the following, as I was trying to rebuild my sstables:

$ nodetool -h localhost upgradesstables
Error occured while upgrading the sstables for keyspace MyKeySpace
java.util.concurrent.ExecutionException: java.lang.NullPointerException
        at java.util.concurrent.FutureTask$Sync.innerGet(FutureTask.java:222)
        at java.util.concurrent.FutureTask.get(FutureTask.java:83)
        at org.apache.cassandra.db.compaction.CompactionManager.performAllSSTableOperation(CompactionManager.java:203)
        at org.apache.cassandra.db.compaction.CompactionManager.performSSTableRewrite(CompactionManager.java:219)
        at org.apache.cassandra.db.ColumnFamilyStore.sstablesRewrite(ColumnFamilyStore.java:995)
        at org.apache.cassandra.service.StorageService.upgradeSSTables(StorageService.java:1648)
        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 com.sun.jmx.interceptor.DefaultMBeanServerInterceptor.invoke(DefaultMBeanServerInterceptor.java:836)
        at com.sun.jmx.mbeanserver.JmxMBeanServer.invoke(JmxMBeanServer.java:761)
        at javax.management.remote.rmi.RMIConnectionImpl.doOperation(RMIConnectionImpl.java:1427)
        at javax.management.remote.rmi.RMIConnectionImpl.access$200(RMIConnectionImpl.java:72)
        at javax.management.remote.rmi.RMIConnectionImpl$PrivilegedOperation.run(RMIConnectionImpl.java:1265)
        at javax.management.remote.rmi.RMIConnectionImpl.doPrivilegedOperation(RMIConnectionImpl.java:1360)
        at javax.management.remote.rmi.RMIConnectionImpl.invoke(RMIConnectionImpl.java:788)
        at sun.reflect.GeneratedMethodAccessor16.invoke(Unknown Source)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:305)
        at sun.rmi.transport.Transport$1.run(Transport.java:159)
        at java.security.AccessController.doPrivileged(Native Method)
        at sun.rmi.transport.Transport.serviceCall(Transport.java:155)
        at sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:535)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:790)
        at sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:649)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
        at java.lang.Thread.run(Thread.java:662)
Caused by: java.lang.NullPointerException
        at java.io.File.<init>(File.java:222)
        at org.apache.cassandra.db.ColumnFamilyStore.getTempSSTablePath(ColumnFamilyStore.java:641)
        at org.apache.cassandra.db.ColumnFamilyStore.getTempSSTablePath(ColumnFamilyStore.java:652)
        at org.apache.cassandra.db.ColumnFamilyStore.createCompactionWriter(ColumnFamilyStore.java:1888)
        at org.apache.cassandra.db.compaction.CompactionTask.execute(CompactionTask.java:151)
        at org.apache.cassandra.db.compaction.CompactionManager$4.perform(CompactionManager.java:229)
        at org.apache.cassandra.db.compaction.CompactionManager$2.call(CompactionManager.java:182)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
        at java.util.concurrent.FutureTask.run(FutureTask.java:138)
        ... 3 more

Any help?

Regards,
Casey