I think Richard will confirm, but I think this is a known issue with Internet Explorer, as I had this problem in December and Richard posted the following:
http://lists.openstreetmap.org/pipermail/talk/2008-December/032364.html
Having said that, I think there have been code changes since to add JS support using ExternalInterface (spotted just in Potlatch.as and World.as) so perhaps it should work. I’ll try and lose some changes here…
Edit:
I can’t get any warning about unsaved changes when switching between Edit and View either in live or offline edit mode using IE8 in both normal and compatibility mode, so I suspect the ExternalInterface amendments aren’t working as desired.
Edit again:
http://trac.openstreetmap.org/browser/sites/rails_port/app/views/site/edit.rhtml?rev=14716
72 window.onbeforeunload=function() {
73 if (!changesaved && !winie) {
74 return “You have unsaved changes. (To save in Potlatch, you should deselect the current way or point.)”;
75 }
76 }
It looks like line 73 deliberately doesn’t give the warning to IE users (and this line predates the addition of the ExternalInterface changes, so may just need tweaking.
Ed