In a recent leak of Microsoft documentation of Windows Phone 7 we learn a few technical bits about the operating system. To start, we confirm that Windows Phone 7 is based off of Windows CE 6.0, meaning that it has a 32-bit kernel and thus can addresses maximum 4GB of RAM at the most (it would be a difficult to argue that a smartphone would need more than that anytime soon, so 4GB seems reasonable).
Also in the documentation we learn that you must enter a Windows Live ID into your Windows Phone 7 in order to use it (this is not good point). This is more strict than Android and iPhone which require you to enter a registered account only if you’re attempting to use the application store. It’s likely that when Windows Phone 7 comes around, Zune Pass users will be able to associate their Zune account with a Windows Live account so you won’t need to sign into two accounts.
In terms of application development, there will be two types of apps: those that are “light” and web-based using Silverlight (think Twitter clients, etc), and those that are more heavy (like games) that use XNA. Those coded in XNA will also be cross-platform apps, being able to run on Xbox, Zune, and possibly the PC (with a bit of work).