This was particularly infuriating back when browsers would commonly lose input state when going back then forward. For years, long after that was resolved, I still wrote anything longer than a single sentence in notepad.
Even if this is kept for inertial reasons (I do make use of it on a trackpad myself), I think as soon as you interact with any kind of text input it should be disabled for the page.
To give fair warning, it seems Safari on iOS 7 will randomly refresh text boxes on pages when you jump to other tabs to cut and paste information. It has bit me a few times and I now wait until I'm at my PC to write any reference laden post or form where information comes from multiple tabs.
Stack (Overflow|Exchange) put up a modal dialog if you started writing something and then change the page. Which is annoying when you just want to navigate away because you're not going to answer anymore, but in most cases solves this problem quite nicely.
Disabling the key under certain circumstances is usually a support nightmare when trying to explain people that »Backspace goes back to the previous page, except there is text in any input field on the page that you entered, in which case it does nothing. Oh, but the back button in the toolbar still works, as does the side button on your mouse.« (The same problem with disabled UI controls that usually do not say why they are disabled. Some things, e.g. Copy being disabled when there is no selection, are easy enough to figure out but others not so much at times.)
Even if this is kept for inertial reasons (I do make use of it on a trackpad myself), I think as soon as you interact with any kind of text input it should be disabled for the page.