When you say "form data" are you talking about the content of posts and PMs? I use Chrome and I've never had a problem with losing content if I hit the back button.
Yeah. Form data is lost with Firefox and IE when you hit the back button. I think this is the default behavior with HTTPS sites.
Weird. I do get warnings with some sites that form data will need to be re-sent and others (like my bank and some government websites) won't let me use the back button at all, but I assumed that must be server side because it only happens with some HTTPS sites.