Löste det själv genom att tänka om. Detta var min lösning:
<div style="clear: both;">
<!-- Översta kanten -->
<div style="clear: both;">
<div style="float: left; width: 22px; height: 22px; background: url(img/box/01.png) no-repeat;"></div>
<div style="float: left; width: 622px; height: 22px; background: url(img/box/02.png) repeat-x;"></div>
<div style="float: left; width: 22px; height: 22px; background: url(img/box/03.png) no-repeat;"></div>
</div>
<!-- "Mitten-raden" -->
<div style="clear: both;">
<div style="width: 666px; height: auto; background: url(img/box/08.png) repeat-y; background-position:top left;">
<div style="width: 622px; height: auto; background: url(img/box/04.png) repeat-y; background-position:top right; padding: 0 22px 0 22px">
<!-- Och denna div ska vara 100% för att anpassa sig efter parent - Men detta är det som inte fungerar -->text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text
</div>
</div>
</div>
<!-- Nedre kanten -->
<div style="clear: both;">
<div style="float: left;width: 22px; height: 22px; background: url(img/box/07.png) no-repeat;"></div>
<div style="float: left; width: 622px; height: 22px; background: url(img/box/06.png) repeat-x;"></div>
<div style="float: left; width: 22px; height: 22px; background: url(img/box/05.png) no-repeat;"></div>
</div>
</div>
