For me, the problem seems to be the "clear:right;" line which has been added to the #main-copy and #main-copy-dark CSS sections. I removed it from both and now all of my pages display correctly again in IE7 and Firefox.
Is this an "ok" fix for now or have I messed up something else?
EDIT: For whatever reason, even though the CSS of these two sections are the same in both the wide and narrow versions, I was only having the problem with the wide version.
Derek