I was just listening to fellow Brit. James Strachan talking about Java Business Integration (JBI) on TSS. In the interview he talks about JBI's role in SOA, the relationship between BPEL & JBI and gives a fairly good overview of JBI. It seems the interview was done just after JavaOne 2005 so may be a little dated but it's definitely worth a listen.That reminded me of an interview I read a while back with Sun's Ron Ten-Hove (Ron is the co-spec lead of the JBI spec. aka JSR208) - another good article to read if you're new to JBI. Serendipitously, that lead me to take a look at Ron's blog which has a great collection of more in-depth essays on JBI.
So if you're interested in learning more about the role of JBI - that's the next couple of hours taken care of :)
If you do have more time on your hands you may want to actually download and play with the Open ESB Community Preview which runs on the J2EE 1.4 SDK (App Server 8.x).