Having been developing mostly in a bubble for a few year, I forgot that some people still use Internet Explorer for Windows. (Sheesh, get Firefox already.)
Turns out (thanks, Tantek!) that IE6 doesn’t support XHTML. That’s not usually too much of a problem, unless you try and make a previously non-empty tag, like <script> empty.
Well, IE6 sees that and dies. It stops parsing the page. So, people using IE6 saw a blank page instead of the RSVP page.
The fix is to go back to using <script>...</script> .