Sprint, Verizon, and US Cellular refuse to unlock their phones, and even if you convince someone to do it, they won't activate a phone that wasn't originally bought on their network. So I doubt that's going to happen.

This right here is why I loathe Sprint.

I actually get a pretty decent price from them for running an iPhone 4S [my employer offers a discount for sprint services], but I'd like to move it to T-Mobile as they would offer a much faster network. Sprint's 3G is terrible here in SE Wisconsin.

Of course: Sprint won't unlock my iPhone, even though it's the exact same hardware that Apple/T-Mobile would sell me. Selling my 4S, of course, means losing my ability to jailbreak.

(I don't plan to move to a newer iPhone, as I hate the form-factor of the iPhone 5... but I also don't want to spend another $600+ on an Android or Windows flagship phone.)

Verizon will unlock the GSM part of your phone if you tell them you're travailing internationally. Would Sprint do that too?

They do have an "international unlock team" that handles this exact scenario. However, everything I've read indicates that the phone still won't accept SIMs from domestic GSM carriers.

If you're jailbroken, why not baseband unlock too?

I have the 4S and iOS 6.

I wasn't aware of any baseband unlocks that work w/ that setup, but it's certainly something I'd consider.