That sucks.
Not that I'm getting an iPhone 4S, still got a year left of my contract, but this would imply that the next iPhone won't have this switch either.
3G reception is no where near as good as GPRS, especially indoors. In my apartment, I turn 3G off because if I walk around during a call with 3G on, it'll drop, where as if I turn it off and use GPRS, it's fine.