If you have the most recent version of ECJ, and want to do checkpointing, I stupidly didn't make ThreadPool serializable so you'll get an exception thrown.
It's a very easy fix. In the file ec/util/ThreadPool.java, just change the line
public class TheadPool
public class ThreadPool implements java.io.Serializable
It's now out on SVN. Thanks to Brendan Poole for that one.