Many of these email scams involve a hijacked account being remotely accessed and Gmail's new feature addresses specifically that - remote unauthorized access.
The most common scam - one we've heard about more and more recently - involves someone hijacking an account and emailing that person's friends and family saying that they're traveling, can't access their bank account for some reason, and needs to borrow some cash.
The thing is, unless you somehow ferret out the fact they they're not who they seem - from contextual clues - there's likely no way you would ever know it was fake. The email comes from the correct email address because it is, indeed, sent from that email address. And the person with the hacked account is likely to be completely unaware.
Google's security feature should help with this, as long as your friend and the scammer don't happen to live in the same area. According to Google, the feature uses logged IP addresses and compares them, over time, to determine whether or not suspicious activity might be occurring. So, if your friend's account is accessed twice within a couple hours, from Colorado and then Mexico, it will alert them.
Within the same window, you will be able to change the account's password. Google Apps customers, the blog says, can look forward to this feature soon as well.