The XMLJunitMultiplesResultFormatter requires having ant-junit.jar in the lib directory of ant. It is used to process multiple outputs for the same test (so can have jboss and java serialization output files for the same test).
I don't know anything about Eclipse (or what/how .classpath and .project are doing), but hope the above helps.
It does help. The .project and .classpath files are all correct and do seem to setup the project's build path correctly. The problem is the built-in ANT that comes with Eclipse probably doesn't have this optional jar. I'll have to put it in there myself probably or just add it to the project's build path (thus augmenting my local copy of .classpath).