On one hand, I agree with mjs; if this has to be run locally then the amount of users you could exploit is limited, since very few people would download a html file, and would not see why they could not just view it online. So its not exactly a high severity issue.
Having said that, I do understand that we can't just tell people that not only can they not open .exe and .com files, since they're executable, but they can no longer open Office (.doc, .docx, .xls, etc) or .pdf files from untrusted sources since they could very well be exploits, and
now they can no longer download/open .htm/.whatever_is_rendered_as_html_in_safari as well. It really is unnaceptable; soon we'll be trying to tell everyone that they can no longer download any executables or data since it would be dangerous, and call this 'user education'.
And even its not automated, neither is XSS, and XSS has become a significant issue, which people agree should be fixed. We still discuss exploits on Office, etc, so I don't see why this should be any different.
So what am I trying to say? Apple should fix things, so should Mozilla/IE; there should be no apps relying on this functionality, and even then, its probably an acceptable loss.
Is it going to be readily exploited? Not yet, but that shouldn't be our criteria for deciding whether to secure things or not.
Oh, and Gareth, you might need this: [
www.crypto.com]