First, the good news. There is a new option on the ISO tab to "Relax Joliet Restrictions - Allow more than 64 characters for Joliet names" Using this option I was able to burn filenames of up to 108 characters total (including the extension).
Now, the bad news. Nero doesn't warn you when it needs to truncate one of these extra-long filenames. For example, when I entered a 128-character long filename, Nero quitely truncated it to 108 characters. This might be somewhat excusable in the situation where one has chosen to relax the Joliet restrictions - if you dare to go over the standard, what you get isn't guaranteed.
However, an unequivocal problem is that Nero doesn't warn you even when you don't relax Joliet restrictions. In this case Nero quietly truncated my 128-character filename to 64 characters.
Nero 5.5 at least warned you when it was truncating filenames (both by changing the font in the compilation display to italics and by displaying a pop-up warning box), and told what files were being truncated.