With the advent of Windows Mobile 5.0, and the new developing tools - Visual Studio 2005 Beta 2 - the average mobile device developer is going to have a few pain points in the not to distant future. When something like Windows Mobile 5.0 comes along, packed with new features, it's always difficult to know how much to focus on the new platform, and how much to keep working with existing devices. After all, currently there are no real Windows Mobile 5.0 devices out there as I write this.
I can't help you decide how to plan your development story, but I can certainly help you plan which tools you need to be using. Or, more accurately, I can point you at the most helpful blog entry I've seen on the topic. Go visit, and print out the very useful Grid Of Devices and Tools for your fridge, and soon you won't be confused.
As for which mobile platform to support - once you try Windows Mobile 5.0 and Visual Studio, you will have a very hard time going back to eVC, put it that way. Writing code that uses the new Windows Mobile 5.0 APIs would be difficult to get working on older devices. Of course, according to plan, pretty soon Windows Mobile 5.0 will be the standard platform - and by using the tools now you are going to have a head start. Heck, I would not be surprised if you had your applications completely finished in time for real devices - which isn't something that happens very often in my experience.