I had the same troubles with the jars, it was the physics one on 2 different machines I tried the demo on. Re-running the webstart solved that both times. It died for other reasons later, but I do have a 3 and a 5 year old graphics card.
Endolf
well I set the jnlp mime type on the server and made a couple minor changes in the jnlp so hopefully itll clear up the downloading problem. I also added option to turn off reflections and shadows for older cards 