There are some problems with running applets that use 32 bit jni libs in Safari (64 bit by default). It could be easily overcame by setting the "open in 32 bit mode" in Safari application "Get Info" dialog but there's no way to switch it programmatically. So, those who are using 32 bit only drivers or codecs in their applets cannot move to Snow Leopard yet.
Joined: Jan 23, 2002
If you need to run stuff on a 1.5 JVM for some reason, beware of Snow Leopard wiping out your 1.5.