Although I've been running Windows 7 betas on and off periodically since the release of build 7000, it wasn't until I used the x64 RC more extensively as my primary OS that I realized Java wouldn't load in Firefox. Intriguingly, however, it appeared to be working just fine in both IE and Safari. I can't say for sure how widespread the issue is, coming straight XP, however, my girlfriend's laptop running Vista Ultimate x64 also faced the issue.
There were no manifestations of an actual error, in fact, Java applets were as blank as could be, and trying to interact with the Java console in Firefox caused the text to gray out with absolutely no result. All the while, Java's plugin was listed in the browser's add-ons, just as one would expect it to be.