I haven't seen this either, just the occasional time out, but the back button works fine in that case. I believe the behavior of the back button and remembering the contents of forms might depend on the browser, so if you're returned to an empty form after you press the back button, you could try a different browser. I also use the crtl+C trick if I've spend a significant amount of time or expect trouble. But I do feel RayJones's frustration when something like that happens, especially if it was a long post.
On a slightly related note, I do occasionally notice a warning about the title being more than 80 characters in a reply (guess the original was 77-80, plus the four characters for 'Re: '). It seems to post it anyway (not sure what the point of the warning is), but it looks stupid. Like anyone even reads the title of replies, it could just truncate it for all I care.