Not just IP address but also browser signature, screen size and resolution, javascript settings and anything else in the HTTP header. See http://browserspy.dk/ for details.
yes, i know (and also browser fingerprinting if they really wanted to), but do they do that? as far as i know, they do not (likely for various reasons - bad press, confusion of people behind NATs, etc).
[i mean, more exactly, that i have heard nothing about them updating personal information with data from searches when not logged in; i am not talking aggregate info like typical searches from geoip locations]