There was a line in that long post on here the other day about Gmail, encryption, and the spam fight that mentioned how what's-his-name explicitly ignored the RFC by scrubbing the IP address out of Gamil headers, so I think you do recall correctly...

Many (most) people connecting to an SMTP server will be doing so from behind nat. Finding out the users "real" ip address is 10.x.x.x isn't so exciting.

Actually you will usually find BOTH the private IP address behind the NAT and the public IP address of the router in the headers. Check out a few mails you've received from a few different sources, see for yourself.

I just picked the two latest I got (and redacted them) as an example.

The first one sent through GMail (SMTP I guess, not webmail) :

  Return-Path: <xyz@gmail.com>
  Received: from [192.168.1.18] (123-123-123-123...bbox.fr. [123.123.123.123])
          by mx.google.com with ESMTPSA id ...
          for <multiple recipients>
          (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128);

and another one, through OVH this time :

  Received: from unknown (HELO ?192.168.0.23?) (xyz@abc.fr@123.123.123.123)
    by ns0.ovh.net with SMTP;

Both "123.123.123.123" were the public IP address from their DSL connections. In both cases as you can see, I could know both IP addresses.

Good point. Looks like google doesn't add these headers if you're using their web interface and apparently the majority of people I email with don't leak this either, however once I started looking there's a surprising amount of emails in my inbox that do. I'm now trying to remember if I deliberately disabled this when I set up our new mail server or not...