It's called the Architect Guide for Windows Phone 7, and it's from February 2010, so it's likely some of these things will change between then and later this year, when the first devices will ship. The Guide contains a lot of information we nerds like to have, so let's take a look.
As we already deduced, Windows Phone 7 runs on top of Windows CE 6.0, which has been out for a while but wasn't used yet by Windows Mobile. It's a major leap forward compared to previous versions of Windows CE. The shell and application platform reside in user space, while the kernel, drivers, file systems, network, graphics/rendering, and the phone update system run in kernel space. Since we're talking a 32bit operating system, it can only address 4GB of memory - 2GB for processes, 2GB for the kernel.
Windows Phone 7 employs Direct3D 11 for 3D graphics - the whole nine yards. This is not some cut-down mobile variant, but the real thing, exactly the same as employed on the desktop. While Microsoft supplies the framework, phone OEMs will have to write the 2D and 3D drivers for their hardware.
Two file systems are supported by Microsoft in Windows Phone 7: IMGFS and TexFAT. IMGFS was already in use by previous versions, and holds the actual system and operating system files. TexFAT is an extended version of the FAT file system capable of addressing files larger than 4GB, and is used to store user files. Microsoft has opted for a unified storage approach, which means that applications and/or users can not distinguish between files in local storage or on a memory card. While this increases simplicity, it also means that crucial files may be stored on a memory card - removing said card will cause the phone to be restricted to making emergency calls only.
The Guide also reveals the specifics of carrier/OEM modifications. While previous versions allowed OEMs to pretty much alter the entire operating system, those days are gone now. They may alter the connection icons and carrier logo, supply wallpapers, and change the boot screen. They may also add bookmarks, change the default search engine (yes), and install a maximum of 6 applications in the ROM. These applications must be free - no trial software.
Updates occur using the Phone Update service, and even carrier updates (e.g., new icons or whatever) must be approved by and go through Microsoft's update service. Small updates will occur over-the-air, while larger updates will use the Zune software and a USB cable.