You haven't posted the exception stacktrace, so i can't say for sure - but I remember that there was a bug where if the jndi name used for injection was incorrect, then instead of a NameNotFoundException, this NullPointerException was being raised from the JndiUtil. This bug was fixed in AS 5.0.
P.S: No matter what search term i try, i am not able to find the JIRA number for this.
sorry, no I didn't post the stack trace (though the position of the null object reference was clear).
There isn't an entry in JIRA yet, I was holding off creating one until someone confirmed that I should do... would you like me to?
The issue is definitely the assumption that the HA JNDI service will be running - if I install the HA JNDI service then that throws a NameNotFound and then I can see where I made my mistake.