I heard about the weaknesses in GSM back in September on Security Now:
It's not eavesdrop-able like the olden days of cellphones back in the 80's when all you needed was a scanner of some sort. But if you want to listen in on a competitor's calls, just acquire the $1000 or so worth of equipment, and set up in a nondescript van next to their office building. Record a day's worth of GSM calls for later decoding.
It's a good idea to practice the "Trust No One" creed when using wireless devices.
"I reject your reality and substitute my own."