I am currently using it with jdk 1.4.1. No problems so far.
We, too, are using JBoss 3.0.3 with Tomcat 4.1.12 using JDK 1.4.1 on both Win2000 and Linux with no problems.
Using it with jdk 1.4.0_02. Haven't had any problems. Iam running jboss4.0Alpha.
The documentation is trying to say JBoss needs
>= 1.3 because it implements j2ee1.3