Excellent - thank you. I tracked it down to this line from product.tpl, in my template:
<div>{include file="customer/main/product_details.tpl"}</div>
Compared to the unmodified version of product.tpl:
<div class="details">{include file="customer/main/product_details.tpl"}</div>
Yikes. Putting that class tag back in there fixed it.
Thanks for your very quick help!