Originally Posted by RedDeMartini

you could have the frame tagged with a transponder or radio beacon but it would be harder to get someone to check than to install it. A cellphone works because you are paying a lot of money for the system to keep track of it. But to ask them to locate your cellphone by triangulating in on it would be a received with more than raised brows. You can get it registered in a county (like Mongomery county Md.) where the bike is recorded on the same stolen vehicle listing as automobiles and thus it gets taken more seriously. But the best thing to do is to install something that will let a mechanic (the only people that will occasionally notice a stolen bike) know who it belongs to. I tell my students to write their name and phone number on the underside of the downtube in very permanent marker or nail polish.
If the bike arouses suspicions (way too big/small-no idea of market value-no idea how to use the SPD pedals that are on the bike), the mechanic can come up with a reason to check someones ID, if it doesn't match they can phone the owner and delay the customer.

(this, by the way is all from long experience).