Eesh, there's so much inconsistency between IE and everyone else with stylesheets.
Setting the margins to 5% seems to have done the trick!
However, my outer div doesn't seem to 'surround' the columns, it put what looks to be a bar right above..
<div style="border:10px solid #c0c0c0;"><div class="column">
But the centering works anyway (Firefox/IE7)-- but how odd!
It's a short code so there's no way for me to have slipped an accidental one in there.
Plus, an extra div would break the entire layout.