So it turns out that this is an issue with Safari when using HTML5's input type="url".
Personally, I think it's dreadful behaviour to stop the submission of the form (although, according to the spec that is correct!), when the form field isn't even a required field.
So if you type nothing in the URL field, the form submits just fine. But if you type something that doesn't match the pattern of being a URL, the form doesn't submit. Madness!
I should probably change the form field to simply be type="text" but that's seems a shame: I like the extra semantics of using type="url" (and I like the adjusted keyboard that gives you on the iPhone/iPad).