This looks like a promising device. http://www.gizmag.com/spybike-gps-tracker/22999/

Just read the web page, that's all I know. If the bike is hidden in a van/other, true you can't get a track on it. But presumably the thief won't leave it out of gps coverage permanently. Once they get out the street again, you've got the ability to track it.

The device notifies via SMS when it detects vibrations.

I wonder how proven this is. I'll look for testimonials...