I've set up a blog using a simple TwentyTen child theme that is identical to TwentyTen except header.php declares the DOCTYPE as XHTML. I've also tried a DOCTYPE of HTML 4.
Anyway, if I use an AddThis "custom/build your own" icons and I choose to use the "Services always to show" option (but apart from this all defaults), all I get on the actual site is a row of red X's instead of the proper icons.
If I choose one of the standard icon choices instead of "custom", the icons display correctly.
What is wrong? Bug?