I get the same thing, and I agree, it's really irritating.

Sometimes it saves the form data and it's still there when I hit "Back", and sometimes it doesn't, and I have to fill the form out all over again from scratch if I hit "Back".

It's especially irritating when, say, the form is a 500-word BBS post, and something goes wrong when I hit submit, and so I have to do it all over again because IE forgot what I'd typed. GRRRRRR.