Agree with what's been said above so far.
I've never had a smart phone before and just picked up the new iPhone 3GS when I started rotations a couple months ago and I love it.
Some questions you need to ask yourself first that may narrow down your choices:
1) Do you want a specific carrier? I was already on an AT&T family plan so it was actually cheaper for me to go with the iPhone, that link above saying the Pre is "cheaper" assumes an unlimited voice plan on the iphone which is more than what I think most people are getting. If want to stick with AT&T then an iPhone or Blackberry (the new Bold is also a very nice unit) are your options. Verizon --> Blackberry, Tmobile--> maybe one of the new android devices coming out soon, or blackberry Sprint --> Palm Pre or Blackberry
2) Prefer a physical keyboard or not? This makes or breaks the iPhone for some people. You really can't go wrong with the physical keyboards on most blackberries. I haven't used the Pre but assume it's fairly decent as well. For me I don't mind an on-screen keyboard, I quickly got used to it and I love the flexibilty of being able to use the keyboard in either portrait or landscape mode.
Do you have a preference as to operating system?
iPhone: Easy to use, polished interface, TONS of apps available, not as "open" to tweaking as the other devices but if you jailbrake that opens up some of the features that people complain about. Probably one of the best touchscreen interfaces out there. Good for audio/multimedia as well
Palm Pre: Interface-wise probably on par with the iPhone. Good contact management and integration with web/online services, multitasking built in. Not as many apps available, but I'm sure that'll change, and you can also run older Palm software through it's emulator. I'm not a huge Sprint fan but it's a very cool device, and probably the only other phone/pda that I would consider if I didn't get the iPhone.
Blackberry: Good for heavy texting/messaging, good email support. Good if you want a physical keyboard and don't need a big screen. The touchscreen Storm is available but I played with a friend's and I was not impressed at all, that clickable screen is just annoying to me, the next generation (storm version 2?) may help make it a better device, but for now I would stay away. Have heard a handful of friends say they took theirs back after a little while of using it. But the other blackberries are pretty solid, I just preferred having a larger touchscreen.
Android: I think there is HUGE potential for this platform. The G1 hardware leaves some things to be desired and it's just a little early for me to fully adopt that operating system as I'm sure most of the main medical apps haven't been ported over to it yet, but I really like that it's an open platform and there should be cool hardware devices coming out soon that use it. Hopefully some of the more useful medical apps will come out for Android.
Windows Mobile: Pretty stagnant operating system, newest one (6.5 I think?) is supposed to come out fairly soon which will be a welcome release. I still have my old windows mobile HP PDA and the thing with windows mobile is it is a VERY flexible operating system in that there are a ton of applications/utilities available for it and if you love to tinker and tweak your device you can on windows, it's just not the most straight forward system to use, plus you will still be needing a stylus at times.
Really, you can't go wrong with an iPhone, Blackberry, Palm Pre, or even a decent/newer windows mobile device. There are good medical applications available for just about all of those devices (not sure what's available for the Pre?) so it's more of an issue of which carrier would you prefer, do you want a full touchscreen or physical keyboard, and what else are you going to do with the device (listen to music, integration with itunes, running multiple apps at same time, customization, etc....)