Upgrading MQ to 6.0.2.11 in the lab seems to have solved it. I can connect from the client now using Java 6 Update 31 with TRIPLE_DES_SHA_US, which was failing under MQ 6.0.2.6
We have a MQ 6.0.2.6 queue manager running GSKit 7.0.4.20 (although I've
also tested with 7.0.4.36 in the lab and had the same problem). We also
have a Java client application that we've been using ...