As Tom said, I think is is a great idea but I'm left a bit confused on implementation and limits.
In my case, I think this would be a nice way to build and maintain forms on a website by using TiddlyWiki as a sort of content editor and exporting things to static pages. I don't know if this plugin would actually support exporting to html but it would be nice.
Also, the instructions only talk about using Google as a back end. What if I want to use my own back end server?
Tie in the forms with something that can read back the submitted XML and display it and you have the start of an interesting sort of open-source InfoPath replacement. Maybe I'm going too far with this but I keep re-thinking how I want the back-end of my still-not-really-ready https tiddlywiki server project.