It is theoretically feasible, although normally used in the other direction by portable devices like the iPad: the device works out its own position with regards to WiFi access points which are in range. In your case, you would have to triangulate for the signals of all systems which emit packets on your WiFi, since you do not know which one is the culprit.
I don't know of any off-the-shelf software or tool which would work in your context (localization of a WiFi user, not of an access point) -- but that does not mean that such tools do not exist.
Of course, nothing can be done for a passive only attacker. To find someone based on the WiFi signal that person emits, well, a WiFi signal has to be emitted by that person.