Forum OpenACS Q&A: Response to IE5 post/get issues redux...
You're right, telling them just to Netscape is not a solution at all. While I lots of reason to believe this is an IE bug, I just don't have the knowledge or resources to determine what exactly is happening. Your best bet is to use Apache or SQUID for reverse proxying instead of nsvhr.
(By the way, here is a Microsoft bug report on another IE bug that has been blamed on nsvhr, but isn't an nsvhr problem) http://support.microsoft.com/default.aspx?scid=kb;EN-US;q175318