In Seam's hibernate-all.jar, org/hibernate/cache/EhCacheProvider.class has references to various classes in net.sf.ehcache, but it's not part of that JAR file. When I look at the latest (6/6/06) version of the source for org.hibernate.cfg.SettingsFactory, I see that the textual reference to EhCacheProvider passed have been removed. Is it a correct inference that the current release of Seam is missing ehcache? I found this because I was trying to write a unit test (using TestNG from Eclipse) and got a NoClassDefFoundError emanating from SettingsFactory.createCacheProvider() called via javax.persistence.createEntityManagerFactory().
That class is only loaded by reflection. So there is no problem, assuming you have default.persistence.properties in your classpath.