Carrier before phone
A phone is worthless if you can't get service. I recommend choosing a carrier that has good reception in the places you are most before trying to decide on a phone. Ask your neighbors and coworkers who they use and how their service is and check the maps the providers list on their website. They may not be exactly true-to-life, but they will give you some idea of the coverage in your area. Once you find a carrier with good coverage, you can find a phone you like.
iOS, Android or Neither?
The next option you'll want to consider is the smartphone operating system. You may have some restrictions here based on carrier, or what your company will support. The currently most popular smartphone OS's are Android and iOS. If you want iOS, there is only one carrier (AT&T) and one phone (iPhone). Rumors abound that Verizon will be getting their own iPhone early next year, but there has been no official announcement to date.
The iPhone is a phenomenon. It really brought smartphones to the masses. Ease of use, hundreds of thousands of apps, and the first phone OS really designed for, not adjusted to, touch input. In my opinion, it's still the most intuitive of the smartphones, and if you don't really like to tweak things, is probably the way to go. The iPhone4 is notable also, for great battery life (for a smartphone), and a high quality camera.
Android has come a long way since T-Mobile introduced the G1 in 2008. It has spread from one phone to many handsets as well as tablets, TV set top boxes, and integration into many other platforms and devices. Based on the same desktop with icons motif as the iPhone and pretty much every other smartphone and desktop OS, most users will find the learning curve for Android fairly easy. Android phones have more options for form factor, hardware keyboards, and user options when compared to the iPhone. Android may not technically have as many apps available as the iPhone, but at over 100,000, it's not really an issue.
Blackberry devices have been the business workhorse mobile for several years, but the Blackberry OS is really beginning to show it's age. RIM has struggled to manufacture a quality touchscreen device, and app support is meager. Many die hard Blackberry users won't give up their "Crackberry" due to the quality and reliability of the hardware keyboards, and security offered by the Blackberry Enterprise solution.
Windows Phone 7
In the last few weeks, manufacturers have released a handful of new Windows Phone 7 devices. This is not an upgrade to Windows mobile, but a completely new OS. Opening day sported about 2000 apps, with most of the most popular types available. There is some nice hardware out there, but being in essence a first revision of a new OS, there are some annoyances such as lack of copy and paste, among other things. I'll be waiting for some updates to see where this platform is headed.
Palm's new OS offering before they merged with HP has been somewhat neglected. Lack of new hardware, and build quality of the available devices is mediocre at best. Currently not recommended. We'll see if HP does anything with this OS going forward.
Carriers and their phones
Top contender phones will be listed for each carrier by operating system. The best thing to do is go to your local store and see what you like.
Android - Samsung Facinate, Droid Incredible, Droid X
Blackberry - Bold, Storm 2
Android - Samsung Captivate
Blackberry - Bold, Curve, Pearl
iOS - iPhone4
Windows Phone - Samsung Focus, HTC Surround
Android - Samsung Vibrant, myTouch 4G
Blackberry - Bold, Curve
Windows Phone - HTC HD7
Android - HTC Evo 4G, Samsung Epic 4G
Blackberry - Curve, Bold