Thanks for the reply.
That would certainly make sense of the error, but the documentation (jvmtiProfilerDoc.pdf which is part of the JBoss Profiler 1.0.CR4 download says Java 5 (in the "Installing the Agent" section).
Other documentation e.g. http://download.oracle.com/javase/1.5.0/docs/guide/jvmti/index.html says that JVMTI was introduced in Java 5.
However, if you say it's got to be 1.6+ for this functionality then I accept that, though as I have to use Java 5 for other reasons it means I'm a little bit stuck.