Indeed, I'd be very curious about this too. Mistake of fact is not always a defense to libel, but I'm unaware of any jurisdiction where truth is not a defense of libel accusations.
Indeed. I remember the phrase "reckless disregard for the truth" from somewhere - possibly the legal definition of defamation, which is pretty similar to libel. You don't have to know it's false, but if you didn't take reasonable steps to verify that it was true, you are still liable (under UK law, anyway). If it's actually true, though, then you're fine.