I had the exact same problem. I use a downloaded font for my headers and on IE8, the Facebook Javascript screwed up the fonts. This occurred when I structured my code in what I assumed was the proper architecture - the Facebook Javascript include was up in my header with the rest of my Javascript includes.
When I moved the javascript include down to the actual div that added the like button, the problem went away.
<div id="facebooklike" style="position: absolute; left: 645px; top: -37px;">
<div id="fb-root"></div><script src="http://connect.facebook.net/en_US/all.js#xfbml=1"></script><fb:like href="http://www.tripinsurance.com" send="false" width="350" show_faces="false" font="arial"></fb:like>
</div>
I think the issue may be happening if the Facebook code is loaded before the the div loads on the page.
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– JM at Work Apr 21 '11 at 08:19