Using ie8's own dev tools it shows the elements text-align value as left, but it is rendered centered in ie8 standards mode. In quirks or ie7 standards, everything is correctly aligned left.
New user, so I can't post images, but check out: http://i35.tinypic.com/2afa9ky.png
text-align is hereditary. Does the parent tag (TABLE, I think) center text?
IIRC, the <th> element will still center and bold the text due to the browser's internal styling. This being a big reason why I've recently started using CSS Reset sheets in all my projects.
Try adding this to your head tag
<style>th{text-align: left; font-weight:normal;}</style>
If you have other tables on the page which you do not want to be affected by this change then you can apply a class to your table and style the elements of that table.
... <table class='work_in_IE'> ...
<style type="text/css">.work_in_IE th{ text-align: left; font-weight:normal;}</style>
A very elegant solution from haslayout.net:
th { text-align: inherit; }