Update: I just noticed on the Facebook social plugin code generate tool, that they now indeed officially support a new HTML5 embed method.
Yesterday I opened Ilikealot and noticed that suddenly it there were two Like buttons rendered in the header instead of one. And it looked weird too. So I went digging in the code. Inspecting the rendered source, and could only come to the conclusion that one <fb:like /> button tag was rendering two buttons. So I removed it. I refreshed. And the weird looking Like button was still there.
Good thought, but I would have liked to be informed beforehand. The <fb: prefixed tags are hard to get through an HTML validator, sometimes mess things up a bit in IE and don't offer friendly support for graceful degradation.
In the end, the solution was simple. Wherever I used to wrap a <fb:* tag inside a <div class="fb-like> .. </div> I changed the classname to xfb-like and that got rid of the problem.
And now I'm just waiting for documentation on how I can fully take advantage of the fb-like classname.