I am currently working a simular problem -- a form that behaves one way on Firefox and another on IE6. (Brokenly in IE, an image object randomly positions itself over the text, the first time you load the page. A reload cleans up the problem, by the way.) The concensus of opinion here is that it is really a (known) IE bug, in the way that it renders XHTML.

The local CSS Gurus have pointed me at something called "the 'quirks mode' in IE". It's triggered by "either the lack of a valid DOCTYPE/DTD or the presence of DOCTYPE on any line of the document after the first". It seems that IE6 is attempting to correct for a broken/missing element (created by early versions of MS Web-Authoring 'tools') and doing it badly.

I spent most of this morning making test versions of my XML document stream and feeding them to the browsers. So far I haven't hit the right incantation, sigh. (XSLT really is a four letter word.)