Boy Genius Report posted some details on the Windows Phone 7 architecture:

Windows Phone 7 is a 32-bit OS with a dual layer architecture comprised of a kernel layer and a user layer. Application processes are given up to 1GB of virtual memory with a total of 2GB of memory allocated to processes. 2GB is given to the kernel. Microsoft will supply the 2D graphics and DirectX 10-based Direct3D 11 runtimes while OEMs, not Microsoft, will develop and distribute the drivers for both the 2D and 3D graphics. Support for Bluetooth 2.1 is included, but apparently support for 3.0 and 4.0 is not. Presumably future updates will be able to provide support to further updates to the standard.

Windows Phone 7 OS will use two different file systems: IMGFS for system files, and TexFAT for user files. User files can be stored on memory cards or internal flash memory and have the system treat both locations as the same. Oddly enough, if a memory card containing key files is removed or the files are erased, the phone will be rendered useless and will be available for emergency calls only.

All Windows Phone 7 updates, including carrier updates, will be distributed through the Microsoft update system. Updates will be disseminated OTA or through the Zune Software installed on a PC.

Windows Phone 7 is tightly integrated with Windows Live ID in a manner similar to Gmail is to Android and iTunes is to iPhone OS. A Windows Live ID is the gateway to the Marketplace which just so happens to be the only place one can go to get applications (side-loading is not supported). The Windows Live ID must be entered in order to use the phone and is required for data synchronization to and from Microsoft's servers. The device must also have a Genuine Windows Phone Certificate which is necessary to authenticate the device.

Changes allowed to the UI are minimal. Providers and manufacturers can add custom tiles to the home screen, but the standard Microsoft tiles can not be removed. The boot screen can be changed by carrier or manufacturer and ringtones and wallpapers can be added. Additionally, Bing is the default search engine for the device, but manufacturers and carriers can change the default search engine found within Internet Explorer. The carrier or the manufacturer can also add custom applications to the ROM of a device (excluding trial apps which are forbidden), but these apps must be first approved by Microsoft. Only 6 applications occupying a maximum of 60MB are allowed.