Looking at the full header, the first (nearest the bottom) "Received: from" will show the first IP, in this case 63.231.175.123.
Code:
Received: from [192.168.1.154] ([63.231.175.123])
by mx.google.com with ESMTPS id h13sm356821ybd.9.2011.09.05.21.57.30
(version=TLSv1/SSLv3 cipher=OTHER);
Mon, 05 Sep 2011 21:57:31 -0700 (PDT)