Yes, when I've seen similar it's been due to ns_returnredirect (I think) not returning a full, valid, URL complete with host (and I believe it is supposed to.) I believe ad_returnredirect fixes that.
Alternatively, I've been seeing a bug in IE (and it's definitely a bug) that's not present in Mozilla, or Opera, or Konqueror, where really big POSTs (or something about certain POSTs) cause IE to retry the POST once, and then issue a GET on the damn thing. It's truly bizarre. Sadly, it does seem related to nsvhr when using the nsunix module, and it's one reason I want to migrate nsunix functionality from nsunix itself to some patches to nssock (I believe nssock has a workaround, the "graceful-closewait")
