So, allow me to ask the obvious question -- is there a way to force the iPhone to 1900mhz only? I sure can't find any setting to do so.
Doubtful. And I really dont see why you'd want to. All you'd be doing is cutting down on your available signal, since you'd be going from two available frequencies down to one. Is the 850mhz band that horrible? If you are having signal issues, I really dont think the solution is to reduce your bands down to one.
If you really wanted to try it, I'd find out if you can get your hands on a tri-band phone first and see how 1900mhz performs in your area. 850mhz is newer in North America, but bear in mind that having the two bands is a big part of what gives AT&T extra coverage over T-Mobile (T-Mobile allows roaming onto the 850 band if you have a quad-band phone but they mostly use the 1900mhz frequency.)
Also, you may just not be able to easily hack that. It's a fairly uncommon request to want to REMOVE better signal options.