Using Windows 7 Pro 64bit and could not get Java to work with IE8 and Minefield (64bit). It was I who did wrong all the time.
I used Minefield to look up Java, but got Java 32bit every time (checked in Control Panel). Took a closer look at the Java site, and finally understood they need to "see" IE8 and not Minefield when I test my browser version there.
So I cleaned the caches of both my browsers, and went back to the Java site with IE8 64bit. Suddenly there they were, the 64bit runtimes.
So now the Java plugin works fine with both my 64bit browsers.
PS: Netbanking working fine with IE8. Have not tested with Minefield.