Verizon - will be releasing the Diamond in the next month (pretty much the Touch Pro without the slide-out keyboard), the BlackBerry Pearl Flip (Flip phone version of the pearl) and also has the BlackBerry Niagara in the works (CDMA version of the Bold). Not a lot else for the next few months.
US Cellular - Only regional so they don't get any device for about 6 months after Spring and Verizon. If you like gadgets, you've already left them.
AT&T - Decent current lineup but the biggest advantage is that you can use the Touch HD, Omnia HD, Xperia, etc on it by buying imports. Apple iPhone 3G version 2 will be out around June.
Sprint - Has a decent selection right now. Will release the Palm Treo Pro in March, the Palm Pre in 2Q '09 and that is about it for right now. The Palm Pre is a new device w/ their new WebOS platform that is written from the ground up to compete w/ the iPhone. Finger friendly, 3rd party apps, easy to build for, sliding keyboard, etc. It's Palm though so they'll find a way to screw it up.
Summary...
If you like touchscreens and live in Iowa, go w/ the Samsung Omnia, Touch Pro, Diamond or BB Storm (Storm has some great leaked OS builds that make it a completely different device than those at the store).
If you want a keyboard, go with the BB Curve, Pearl or Touch Pro.