I'm surprised that so many people don't understand why anyone would want this.
While it is indeed a fact that turning off 3G saves battery life it's more than that. In a fringe 3G area, but with a good EDGE signal, the iPhone 4S will almost always try to stay on 3G--even if it is barely adequate. Actually it will do so when it is inadequate: there are reports of calls going straight to voicemail now whereas on prior iPhones, with 3G manually turned off, the calls came through fine.
In addition to 3G using more power it is even worse in the above scenario as it constantly switches from 3G and EDGE.
Also, while the 4S's antenna is improved it has nothing to do with being able to turn off 3G manually and why someone might want--or need--to do that.
Unfortunately, there have been reports that the 4S's baseband is incapable of manually shutting off 3G. I find that hard to accept as the 4S can certainly handle EDGE and even GPRS. So hopefully a software change can restore it.
Michael