The conflict is between the editor style of
WIDTH: 100%;
And something within the roundframe div class.
It seems to be treating the roundframe, if I understand correctly, as the width that 100% is related to.
On further investigation(mostly accidental, as wordwrap disabled made me, after cutting the textarea's style out into a div around it, and that not working, then while adding it back in, I missed that I left the div with an identical style in, and oddly, it worked...), I can come to a semi-probable-somewhat-conclusion that the textarea, on IE6, reevaluates it's own style only partially when sufficiently modified, and probably disregards padding when finding it's width by percent.
If you have an old copy of IE6, try these two HTML codes. The extra-long style is literally a copy of the one in the .css, so that it doesn't rely on external dependancies. The bolded text makes the difference.
<DIV style="BORDER-RIGHT: #adadad 1px solid; PADDING-RIGHT: 1.2em; BORDER-TOP: #adadad 1px solid; PADDING-LEFT: 1.2em; PADDING-BOTTOM: 0.5em; MARGIN: 0px; BORDER-LEFT: #adadad 1px solid; PADDING-TOP: 0.5em; BORDER-BOTTOM: #adadad 1px solid">
<TEXTAREA style="WIDTH: 100%; HEIGHT: 175px"></TEXTAREA>
<DIV style="BORDER-RIGHT: #adadad 1px solid; PADDING-RIGHT: 1.2em; BORDER-TOP: #adadad 1px solid; PADDING-LEFT: 1.2em; PADDING-BOTTOM: 0.5em; MARGIN: 0px; BORDER-LEFT: #adadad 1px solid; PADDING-TOP: 0.5em; BORDER-BOTTOM: #adadad 1px solid">
<div style="WIDTH: 100%"><TEXTAREA style="WIDTH: 100%; HEIGHT: 175px"></TEXTAREA>
In fact, there is already a suitable <div> around the textarea, so simply setting it to style="width: 100%" would ensure that IE6 uses that div for sizing of the plain-text version of the post/modify box.
And, I haven't tested it much, but it might break something on other browsers, so I only reccomend using it if it can be made browser-specific, and then only browsers that are improved by the fix use it, is possible.