There's a lot of options. Depends on what carrier has good service where you are and how much you want to pay. Data plans range from $15 and realistically $25 for the iPhone on AT&T while most other carriers are $30 for their smartphones.
So the iPhone 4 comes out at the end of this month and that's on AT&T. It really has become the standard in terms of apps available for it and especially medical apps though in the past that was held by the old Palm machines.
Many people like the android phones and your options are the new HTC Evo 4G on Sprint which adds the additional $10 onto the data plan for 4G service even if you don't have 4G in your area though Sprint is in general lower cost, Motorola Droid on Verizon, HTC Droid Incredible also on Verizon. Problem is that all the apps aren't there. Epocrates Essentials is still not available for Android.
You can also go with a Blackberry phone which are on most carriers but I do recommend a touch screen. I don't think BB is doing much in terms of work on its Storm series which is a touch screen and screens tend to be on the small end compared to other smartphones. Many of the medical apps are available for the Symbian OS which BB uses.
You can also go with Palm including the Palm Pixi Plus and Palm Pre. They are very low cost phones up front with contract. Many apps are now available including Epocrates Essentials. Problems include smaller screen, keyboard isn't the easiest to use, and uncertain development of the Palm OS in the smartphone realm after HP bought the company.