I would encourage you to look at JBoss SEAM (http://www.jboss.com/products/seam
). It's a web framework that seems to be one of the few to actually discuss the back button issue up front rather than bury it in the documentation.
There is an article on it here...
FAQ Article on back button - http://labs.jboss.com/portal/jbossseam/faq#back
You could also check out Spring WebFlow, although I did find it harder to find a good statement indicating the the back button is not an issue. Spring MVC and WebFlow are both excellent frameworks though, and well worth a look.