It actually depends on how the caching is set on the browser.  You can add directives to your page to try to make sure caching doesn't occur (check this out, and this) -- which will give you full control of what happens when the user pushes the back button.

With IE7 and default settings (my setup) page_load always gets hit when the back button is clicked.

More

http://www.nikhilk.net/BackButtonSupport.aspx