I\'m working with a local HTML5 file -- it\'s got at the top. I put something like this in it:
Turns out it was a namespace issue: the elements need to be created with createElementNS("http://www.w3.org/2000/svg", ...), which Prototype happens to have no built in (new Element(...)) support for. So I guess the HTML5 SVG situation is basically "SVG without a namespace gets the namespace added during parsing (but after that it's just like XHTML before)".
P.S., the "Answer Your Question" button here isn't working for me today, so if anybody adds this as as answer, I'll mark it correct. :-)