IE7 bug - font-style: italic; not harmless?

Here's a strange thing which took me hours to track down. I'd have
thought that 'font-style: italic;' was generally harmless, but
apparently not to IE7 and/or Vista.

The stripped-down version of the web page is at
http://www.wessex100.org.uk/index_demo.html and the style*** is at
http://www.wessex100.org.uk/shared/wessex_demo.css . Both validate.

The odd behaviour affects the width of the page displayed and thus the
placing of the centered background image. Everything works as it
should with WinXP running Firefox, Opera, Safari and IE6, and with
Vista running Firefox. It goes wrong with Vista running IE7.

To make the page display correctly in Vista/IE7 I've found I have to
take out two occurrences of font-style: italic; . The demo version has
these suppressed, but if you take either of them out where I've
indicated you'll see the effect. None of the other OS/browser
combinations I've mentioned are affected.

Is this a known bug, and can anyone explain it?
.