In a way, I kind of did this both ways. I was initially supposed to go in for surgery in April 2007. I really did not want to tell anyone. I told my wife (as she was doing it as well), my parents, and a couple of very close friends, but no one else. Figured the same as you, nobody's business. Well,in my pre-op tests, they found a Softball sized tumor in my chest which needed to be removed. Needless to say, this put the whole Lap-band procedure on hold for a while. So, I needed to have a surgery to remove it, cracked open my chest, 6 week recovery time, etc,etc,etc...
Anyways, the point of this rant is the thing that really amazed me is the incredible support I received from those same people I was trying to keep the Lap-Band secret from. Kind of felt silly that I wasn't willing to share this with them. So, when I did finally go in for Lap-Band surgery in December, I shared it with pretty much anyone who asked.
Give people a chance, I think they can be really supportive. But that being said, you know your colleagues best, and you know your personality. I certainly understand both schools of thought, but have no regrets in sharing it. Hope this helps.