This is surprisingly tricky, but in the end (with a bit of help from the safari java console) I determined that the problem is with DNS resolution and have a work-around. Whether this affects everyone or just those of us lucky to be behind a corporate firewall, I don’t know. The fix for me, was to install Java 1.5 and set my java preferences to always choose the 1.5 JVM for running web apps/applets, after which everything seems to play nicely.
Installing java 1.5 on Snow Leopard is itself quite tricky. The secret is to get hold of the java 1.5 update for Leopard and open it using Pacifist, which allows you to navigate into the /System/Library/Frameworks/JavaVM.framework/Versions directory within the installer package, select the 1.5.0 folder and the symlink to it (1.5) then right click and select “install to default location”. Props to this blogger for revealing the secret to that old chestnut. Please be very careful though – if you don’t delete the existing symlink first (which has 1.5 pointing to the current 1.6 install), then you’ll blow away your existing installation!
Unfortunately this means I no longer have an excuse not to attend boring finance meetings whilst working from home.