I followed the advice on this page of HTC's website: HTC Desire - Backing up phone data i.e. that when performing a factory reset, that SMS messages would be restored. So, I factory restarted my phone and waited for my texts to reappear. When they didn't, I phoned HTC. It turns out that they accept that the information on their website is false. I've lost 3 years' worth of text messages and am absolutely furious. I wanted to try and make sure as few people as possible suffer the same problem.