I just found this website, so I haven't been lurking very long - forgive me for barging in!
Anyway, I have written a Java Swing applet for our intranet, using JDK 1.2.2. It runs in Netscape and IE, on Suns, PCs and SGIs. Yes, you have to sign your applet in order to access any local files (we transfer files back & forth, client/server) Since I am using the plugin I've used Sun's jarsigner tool and downloaded the .keystore & .java_policy files to each clients .netscape dir & home.
Each platform has it's own set of quirks but it can be done!
If you are able to run Swing without the plugin - please let me know how! Then you should sign your jar file using Netscapes signtool.
Oh yeah, I am using a self-signed certificate (I created it)stored in my .keystore file. Rumor has it those will not be accepted in future releases???
Good Luck,