I tried changing your site to a standard theme (several of them) and the buttons are still messed up. I don't see anything wrong with your header either. I'm wondering if this is a jquery incompatibility issue. Try downloading the latest version of jquery and the jquery UI and replace your site's copies. You'll need to change the version numbers on the new files or change the links in your template_top.php to match the files.
Please let us know if this works.
I'm sorry - I was sure I responded to this a couple days ago - I did make the changes you suggested but I'm not seeing any improvement. I have noticed that a few of the buttons DO show up properly. I added a login box and the login button there looks just fine - but some of the checkout buttons are strange looking as well.
Thanks again for your help!