I bought one of these to fit to the new car (we’re going to be driving it all over Eastern Europe over the summer so I thought it a sensible precaution). It arrived yesterday, and I have to say I’m extremely impressed with it. The thing that struck me was that it’s really small (about the size of a cigarette packet) and would probably be pretty easy to hide on a bike as well. Add a pre-paid sim card and you have a decent bike tracker system for under £30!
http://www.amazon.co.uk/gp/product/B003XDN58K
It’s designed to be wired in to the battery, but it has a built-in backup battery so if someone cuts the power wires it’ll still monitor and transmit. You control it all by sending it SMS messages, where you set up “authorised numbers” to accept calls from, and then you just dial the number to have it send you an SMS message with exactly where it is, including GPS coordinates (longitude and lattitude) and a link you can click to go straight to Google Maps. It can also do GPRS if configured, and use that to find out the actual street name it’s on and include that in the message; more interestingly, if the GPS signal is unavailable (e.g. it’s inside a truck) it can use GPRS and location based services to give you its location. You can also put in an SD card to have it store location data if it’s out of Mobile phone signal range, so if it has GPS but no mobile signal then when the mobile comes back it can tell you where it’s been, and you can also download the SD card data via a card-reader once you retrieve it. It can do GeoFencing (sends you an SMS with location when it moves out of a pre-defined area), and it will automatically send you messages if it loses external power.
There are also a bunch of external cables you can wire in if you want to. It comes with an “SOS button”, which sends emergency SMS messages with location if you press it, and if you really want to you can wire it into the speedometer, fuel level, alarm system and engine cut-off, although I think you’d be mad to do this (anything goes wrong and you could find bad things happening – keep it simple!).
All in all, I really am very impressed; if you want to put a tracker in your bike at a very low cost, I highly recommend this 
-simon