We found this problem, where specifically the Add To Cart button was not working in IE for some products, but not all. This was due to the content of the Product Item's Description field which appears before the Add To Cart form.
If the users were using the HTMLAREA editor to enter the description, after a few edits and cuts & pastes the actual content would eventually become convoluted with irregular combinations of code.
We just need to go in and remove all non-text from the description and update the product.
Again, we found this to be true when some, but not all, of our product Info/Add to Cart buttons weren't working. But IE is definitely not forgiving.