Malte, you can do this with CSS and html. No need for added complexity (especially if it introduces invalid html)
As part of the Zen work Mark spent some time testing example forms in all kinds of configurations using fieldsets and he was able to do magical things. I will see if he has those examples somewhere online. In the meantime google is your friend. E.g. a very quick search brought up this page which has two text inputs on the same line, where both the text inputs have a label (hidden via CSS). http://www.websemantics.co.uk/tutorials/accessible_forms/ (2.2 Simple Login)
Carl
