heh?

Just tried it.
1) Go to www.av.com
2) go to gmail -> automatically logged in
3) Click the back button -> Back on altavista page

So, I don't see how it could be.
Except if gmail is the first site you visit with your browser window/tab.
Then the back will have no effect.

And as the back having no effect inside the gmail site, thats another thing.
It's due to the AJAX design of the sites. Look there for an explanation:
http://www.contentwithstyle.co.uk/Articles/38/