I'm writing this up because we're bound to hit the problem again at some point, and it took me ages to figure it out.

Writing a login system for the Roman Law db (which I'll cover in another post), I created a login form, and naturally I thought I'd make it consonant with the rest of the site by causing it to be rendered as a JQuery UI dialog. Once I'd done that, I started testing the actual login functionality. It failed every time; neither POST nor GET parameters were submitted to the server. I didn't twig for ages that this had anything to do with what I thought was the largely cosmetic decision to wrap the form components in a dialog call, but apparently that was it; as soon as I removed the JQuery code, all was well. I haven't yet been able to track down what exactly it is that causes the dialog to render the form ineffective, but I know to avoid it in future.