Information sent in headers through the GMail web interface:
Received: by 10.0.22.202 with SMTP id o10mr1683492bkb.70.1307695606700; Fri,
10 Jun 2011 01:46:46 -0700 (PDT)
Received: by 10.0.57.83 with HTTP; Fri, 10 Jun 2011 01:46:46 -0700 (PDT)
However, using a software client, my email headers say this:
Received: from [192.168.17.21] (myhost.myisp.com [188.8.131.52])
by mx.google.com with ESMTPS id ex2sm1432764wbb.65.2011.06.10.01.32.47
Fri, 10 Jun 2011 01:32:48 -0700 (PDT)
So yes, your email headers may contain the IP Address of the location from which you connected.
Can you deduce a hardware MAC address from this? Not unless you're on the same subnet and can send ARP requests or have some other local access. That's how internet routing works - even Google's SMTP server won't have had that information. If you do have local access to a network, using email to get that information is probably excessive anyway; you could find it simply by listening on the wire or OTA.