I personally always expect 'back' buttons on multi-page form processes. Having a button to move forwards without the equivalent control to move backwards is disconcerting. As you note, the browser back button won't always work with forms, so some people are nervous about using it.
You'd also be surprised (as I was the other day when presented with the results of a user testing session) how many users don't use their browser back button. They expect to find a link within the page.
One potential issue with form processes is how you handle validation when a user clicks your 'back' button. If they have modified fields on the current page, do you save that data? If they have not yet completed all mandatory fields, do you force them to complete those fields before allowing them to go back?