Here's an example of spam and how the "from" line can forged to show
your address - I found this today from my semi-throwaway gmail addy (it never made it to my actual inbox - GMail stuck it in the spam folder:
Delivered-To:
(me)@gmail.comReceived: by 10.142.141.1 with SMTP id o1cs175975wfd;
Fri, 21 Mar 2008 17:15:40 -0700 (PDT)
Received: by 10.65.20.18 with SMTP id x18mr7760746qbi.48.1206144939999;
Fri, 21 Mar 2008 17:15:39 -0700 (PDT)
Return-Path: <duncanschofield@iprimus.com.au>
Received: from d610geo (206-248-131-220.dsl.teksavvy.com [206.248.131.220])
by mx.google.com with SMTP id c5si3523917qbc.19.2008.03.21.17.15.38;
Fri, 21 Mar 2008 17:15:39 -0700 (PDT)
Received-SPF: softfail (google.com: domain of transitioning duncanschofield@iprimus.com.au does not designate 206.248.131.220 as permitted sender) client-ip=206.248.131.220;
Authentication-Results: mx.google.com; spf=softfail (google.com: domain of transitioning duncanschofield@iprimus.com.au does not designate 206.248.131.220 as permitted sender) smtp.mail=duncanschofield@iprimus.com.au
Date: Fri, 21 Mar 2008 17:15:39 -0700 (PDT)
X-Originating-IP: [206.248.131.220]
X-Originating-Email: [(me)@gmail.com]X-Sender: (me)@gmail.comReceived: (qmail 2180 by uid 510); Fri, 21 Mar 2008 08:15:37 -0500
Message-Id: <20080321031537.2182.qmail@d610geo>
To: <(me)@gmail.com>Subject: RE: MedHelp 73816
From: <(me)@gmail.com>MIME-Version: 1.0
Importance: High
Content-Type: text/html; charset="ISO-8859-1"
Content-Transfer-Encoding: 7bit
Either that or I'm going crazy and sending myself spam!!

(Interesting that the supposed originating sender is "duncanschofield@iprimus.com.au" - the user ID is somewhat similar to my Google ID; not sure what THAT means, if anything!)
I did try looking up the IP address that GMail's servers complain about - at first I couldn't find it. Tried again on another site and it's actually a Teksavvy.com IP - they're a provider here in Toronto. Unusual, in that they're usually offshore. When I did a Google search, I got one result:
http://en.wikipedia.org/wiki/User_talk:24.118.187.54Apparently this IP has been used for various nefarious purposes!!
Even though SpamCop would report this to TekSavvy.com's abuse, I'm not going to send it on since it says it comes from ME at my gmail addy, and I'm darned if I want any confusion!! I don't generally bother reporting the Gmail spam via SpamCop, since I don't really completely trust the abuse departments at some of the less-than-stellar offshore ISPs. Not that I think TekSavvy is scuzzy - they actually have a pretty good local reputation from what I can tell. I do occasionally have a go at SpamCop with my Yahoo spam, because that REALLY is a throwaway account and I don't care what happens to it.