If you want a completely unlocked phone, compatible with all US carriers, with insane* battery life and a 3.5mm headphone jack, there's only one option I know: The $500 Motorola. That's why I bought one.
Only today, you won't have to pay $500. The phone's currently on sale for just $350 directly from Motorola.
Mind you, it's not quite as good a deal as when the-- its faster, shatterproof sibling -- was on sale for just $300 at T-Mobile. But that would have locked you into GSM carriers and it doesn't have a I never really considered it, personally.
*For insane battery life, you'll need to buy one of Motorola's snap-on magnetic Moto Mods battery packs, which add a little bit of extra bulk to the phone. I'd recommend the $80 TurboPower one if you can find it -- it's grippy and charges up fast with Motorola's adapter. Without, the battery life is still better than most phones you can buy, but not exactly groundbreaking.