AT&T: In cahoots with Apple in a major way and the remainder of their smartphone focus is taken up with the Bold etc. They're not gonna risk upsetting Apple, so no Pre for them.
Verizon: They've passed entirely on the 800w and the CDMA Treo Pro and been late with every other Palm device EVER (with the exception of the 700w & 700p, and both of those were widely panned by everyone here). I think they are looking to focus on feature-laden dumbphones like the Dare(cheaper/easier to support) alongside their usual BB and WinMob efforts. So they're probably the best bet to get a Pre but I am NOT counting on it.
Alltel: Being assimilated by VZW as we speak, so they're out too.
T-Mobile: Haven't carried a Palm device since the Treo 600. The Centro would've been perfect for them (colorful, cheap, popular, small) but they pased on it. I don't see anything changing with them even if the Pre ends up as a smashing success.
That said, I am certain Rogers or Bell in Canada will end up with the Pre as will the usual overseas carriers. But for the US, I really forsee only Sprint carrying it...at least until WebOS has eastablished a strong market presence.