I'd also like to add that I plan on doing some light programming for the phone.
I would say Android is best for this purpose... the entire OS is open sourced (or at least the vast majority of it), and all apps are written in Java. Google provides the "NDK" that allows you to write some native C/C++ code, however it's advised most of your code be implemented in Java.
Of course the iPhone SDK is readily available as well, however iPhone development requires an OSX install, which is a limiting factor if you don't own a Mac or an easily Hackintosh'd PC. As far as I know, iPhone apps are written in Obj-C, however I've never played around with the platform.
I believe the WinMo7 SDK is already out and that is all done in C#, and at this point MS is not providing any means of writing native code. Microsoft does, however, allow XNA programs to be compiled for WinMo7 which makes it very easy to program 2D/3D games for the device.
WebOS and Blackberries are less familiar to me so I can't comment on their development environments.
As I mentioned I feel the Android platform is the healthiest from a developers point of view, as far as options and simplicity. iPhone has a larger marketshare at the moment, so if you want to market your app there's a wider customer base. Microsoft on the other hand has always, IMO, provided very intuitive APIs and mature development tools for programmers, however they are also known for less open environments.