I missed this last week; JBoss ESB version 4.4 was released last week. Lots of bugfixes. Release notes are here and downloads are here.
It's incredible how many open source ESBs are out there right now. Apache has three of them, JBoss has one, WS02 has one, and then there's Mule. There seem to be more open sourced ESBs right now then open sourced web servers even with Sun's recent announcement that they are releasing Sun Java System Web Server 7 with an open source license.
If anyone uses JBoss ESB, I'd love to know what you think of it.