I'm looking at getting one of these:
Sony Ericsson k750i

What is the best carrier that will:
a) Activate my phone, and
b) Let me pay month-to-month, terrestrial phone service-style

Well, that's a tri-band GSM phone, so by default, Sprint and Verizon are both out since they're CDMA. AT&T is out because most of their network is 850MHz GSM which that phone doesn't support, so except for a few select areas in the US (Rolla NOT being one of them) you would not get service at all. That leaves T-Mobile, who is 1900MHz GSM, and it WILL work just fine on that phone. the only problem? Roaming. T-Mobile roams on AT&T out in the boonies, and AT&T is almost completely 850MHz, so if you went out into the boonies, you wouldn't have any service. T-Mobile has coverage in Rolla, as well as all along the major interstates in MO and the big cities. If you go to T-Mobile's website, you can look at a coverage map and see if they have coverage where you need it. And if they do work for you, T-Mobile has this thing called FlexPay which is a monthly payment that doesn't require a contract. You'll just need to sign up with them and get a SIM card and you're all set.

Any more questions, let me know. Cell phones are one of the few things I actually know a lot about

Sweet! That's exactly the kind of info I was looking for. Thanks!

EDIT: So, a quad-band phone like the Sony Ericsson W810i would be a better bet, coverage-wise?

Yeah. The w810s are great. I had one but upgraded to a smart phone. But it was an awesome phone....my brother has it now and it's still working great.

I'd still stick with T-Mobile though....I had AT&T for 2 years and they were shit. I'm with T now and they're great.

Yeah, I meant get the W810i and T-Mobile and still be able to roam on the 850MHz networks where needed.

Anyone ever have US Cellular? I am tired of Cingular/AT&T being a pain in my ass and would like someone different. US Cellular seems to have better coverage (from THEIR map vs AT&T's coverage map) and I need something that has a national plan...

My girlfriend had it for a while. She moved to Rolla and found that in most of the town she had zero coverage (but, this was like 2 years ago). However, she says their customer service was really good. She told them she had no coverage here after their rep promised her she would, so they let her cancel her contract with no fees attached (this was after having their service for something like 3 months).

If they actually have good coverage here now, I'd say it wouldn't be too bad.