PocketMatrix

from http://msdn2.microsoft.com/en-us/library/ms839548.aspx
Pocket PC and Smartphone Share A Common CAB Format
Windows Mobile 5.0 software for Pocket PCs and Smartphones now share a common CAB format. Sharing a common CAB file format notably reduces the time required to create CAB files and to support the installation process because all device installations are handled through a single CAB.

I have several questions about this:
1) Do you need to have Visual Studio 2005 to build this CAB file?
2) Is the CAB file compatible with older OSes such as WM2003, WM2002, WM2000, SP2003?
3) Is the CAB file actually smaller or is it just a combination of 2 totally different installers?

1) Probably, since VS creates the inf file for you now.
2) Pass.
3) Yes, I think it's one single installer. Fundamentally, the platforms are pretty much identical - I think it was actually the Smartphone that moved to controlling app installation on the device rather than through Activesync.