I assumed that they were bluffing about shutting down my site, since it appeared to be working, but I considered whether it was worse risking the child porn thing and decided that it wasn't worth the inconvenience of potentially being investigated.
This morning, I discovered that the hosting company who provide this site had in fact suffered a massive DDOS attack (this is where a few hundred computers from around the globe are directed to bombard their server with requests and other traffic non stop). To protect themselves, they had to temporarily close down my site. The timing of this attack was exactly when my correspondant had told me that my site was already shut down. The target of the attack was my site alone. In other words, there was no empty threat.
The child porn threat was also possible. Emails can be faked to come from anyone. Indeed, when I send an email, apparently from me, it goes through various servers, none of which are directly linked with my site. I could send an email from my email address using any server in the world. How do you know an email has genuinely come from me? You don't. So, someone deciding to send out a few hundred thousand child porn images, appearing to come from me, could easily make it look the same as if I'd really done it. Though the authorities would have to work hard to prove it really came from me, they'd at least investigate... police cell, perhaps? scanning my hard drives? watching all my DVDs and videos to be sure they're really musicals, Disney and Lost, rather than some well-hidden kiddie porn... quite frankly it's scary what a malicious person on the internet could cause to happen with only a few keystrokes.
I emailed the guy last night and told him I'd sort out my site today. He emailed to apologise for the inconvenience (!) and I guess I should expect no further problems. I've modified the blog entry as he requested (removing the name of the company he claims to come from) and my website is now back online.
My freedom of speech seems to come at the cost of a lot of inconvenience. Though I had done nothing wrong, the risk to my hosting company of hosting my site would have been too great had I not made the change. Another attack like this and they'll terminate my contract, though they've been very reasonable and pleasant with me about it (and efficient). In addition, though I have not done anything vaguely spam-like, let alone involving porn, the personal stress of being subject to investigation would be more than I think a facetious remark on a website is worth. So, I censored myself.
Welcome to the real world.