By that time Java 6 will be one year old and even Java 7 will probably be around. I would start dropping support for 1.4 in jBPM 4.
Many people is already running jBPM on Java 5 VMs and more will over time, so it should be a smooth transition by '07.
The web services project is written in Java 5. This project makes its code runnable in 1.4.2 using JBoss Retro. With Adrian working on it, it'll most probably be quite reliable :-)
In the company I work for we just migrated from Bea 6.1 (jdk1.3) to Bea 8 (jdk1.4) In contrast to JBoss, Bea only certifies its appservers to run on certain jdk's. So lots of people who do not use JBoss could be on older jvm's. I think we/you could write against Java 5 and use Retro to enable it to run on 1.4.2
JSE 5 is the way to go. I think the industry needs a little push to get going on this and JBoss can be part of the leadership in this respect.