The idea is that when you register the address email@example.com you will also get emails sent to firstname.lastname@example.org and the johndoe address will be reserved.
GmMil doesn't recognize dots (.) as characters within a username. This way, you can add and remove dots to your username for desired address variations. messages sent to email@example.com and firstname.lastname@example.org are delivered to the same inbox, since the characters in the username are the same.
Nice idea and potentially useful, there are other little tips like using the email address email@example.com will have the email sent to firstname.lastname@example.org and the +astring can then be used for filtering into folders. (See this site for more info)
Unfortunately it appears that if someone else had already registered email@example.com they still keep that email address, but their emails get sent to your account as well, or at least that is what some people on Slashdot are saying is happening.
I've had an address in the form firstname.lastname@example.org for a long time. I picked it because I liked the separation. I don't even know if email@example.com was taken before I registered. But I know that someone else (named Dave) has firstname.lastname@example.org, because i receive his messages, including ones with private information, like web site admin login/password combos, which would give me access to billing information.
People argue if this is a bug or a feature. Its just something worth keeping an eye on.
Update: I got curious and tested this (I know I should have done so before reporting this :-) ). Anyway for comments on this blog I have email@example.com (currently being filled with spam). I just tried to register decs.ramble and was told it wasnt available. So it would appear the problem is fixed for new users, but older users may have gotten accounts before it was fixed.