I have had my first go with WEC7.  Here are some brief initial comments:

• Don't install the CTP on your main CE 6 ( and probably CE 5) development system.  On my system CE 6 wouldn't work, even after some reinstalltion of CE 6.  Official word is side by side isn't supported.
• There are some useful documents installed in
  C:\Program Files (x86)\Windows Embedded Compact 7\Documentation
• The emulator is an x86 VPC.  With Win 7 you can use its VPC, otherwise need to install VPC 2007.  Because Win 7 VPC doesn't have virtual floppy (you use the emulator like a CEPC ..ethernet connection) you need to use teh supplied virtual hard disk.  See the documentation in the directory as above.
• Remote tools are better integrated.
• The subproject files are slightly different.  Haven't figured out what the difference is exactly.  For my stream drivers etc I craeted new subprojects in the WCE7 context and imported my original source code.
• The architecture hasn't changed much.  Its more about additional features to the OS, rearrangement of some of th folders and some tool improvements.
• There is an issue with signing binaries that added post build to an existing system.. See the documentation.
• Qusetion Is Windows Phone 7 CE R3 or a WEC7.. I suspect teh former.

Cheers djaus