I really shouldn't blog when I'm angry or annoyed as I tend to rant a little more than I intend! In retrospect I was a little harsh in my last post -- anyone who freely gives their time to developing free software shouldn't have to put up with me disparaging their work.
So as penance I've now tracked down the source of the weird class loading bug I highlighted and have submitted a detailed bug report, including a proposed fix, to the IcedTea netx project (netx is the name of the open-source Web Start replacement). You can monitor the progress of the bug through their public bug tracker. If I had the right permissions it's such a simple fix that I'd be happy to do it myself, but you have to earn the respect of project maintainers before getting the right to commit code.
Update, 23th February: it's now been fixed in the main code tree although it will take a while before it makes it into an Ubuntu update.