Pretty decent post on using cell phones to bug. Or rather how to notice if it is happening.
The key is that for some phones it is possible to dial to them and turn them on without letting them ring. There are a lot of different ways to do it. In most cases (but not all) you have to have physical access to the device at least once.
Mike Larsen did some stuff on this back in the 90's I dug it up on packetstorm this weekend.
My Last gig was a major oil company. I spent a bit of time working with a few people in legal and acquisitions explaining this exact issue to them. Not because I was pushing it but because on more than one occasion it was run into. Always when overseas visiting a more ... lets say intrusive government. Actually several different countries.
It is more widespread than you might think. We caught it a few times usually when the other side was sloppy.