There's a fellow in Iowa using an Arduino as part of a GPS guidance system he developed for his tractor.<p><a href="http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1254445743" rel="nofollow">http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1254445743</a><p>Last I checked, he had his variance from ideal track down to 1.5 inches.  Interesting read.