Gmail has a back button because Google uses AJAX heavily to update the same page over and over again. Since Stack Overflow leans more towards Web 1.0 and reloads the page when you navigate, there's no need to waste time with such a feature. Just use your browser's back button.
Should Stack Exchange ever move to a model whereby there is a single page whose content is replaced each time, then it would make sense to have an application back button. However, since AJAX is only used to add in new answers and comments, the browser back button is sufficient. When you move to a new post, the page reloads.
In short, an application back button is by no means a required feature and would be a waste of development time.
Note that, while Gmail may not require a back button, (good points @MarkTrapp), the goals of Gmail appear to be more geared towards acting like an application, not a Web site. We still don't need one here, and the argument against still stands, for now...