After you download and unzip the JVM, you will need to install the following 3 PRC files that are in the ARM4T folder:
J9JavaVMMidp20.prc
j9pref.prc
JavaVMCheck_enUS.prc
Opera Mini has a couple of quirks (at least on my 755p) -- when it asks permission to connect to the internet (with the standard Palm dialog) it is really tiny and up in the corner. I just press the select center button and it connects, but the connecting dialog is off the bottom of the screen. After it connects, it all seems to work OK though.