Since we're on the subject of new builds and how WEH fits into it, I thought I'd elaborate a bit on the future of our aging OS.
Windows Mobile 6 is in the end of its life. It reached the beginning of its end of life cycle in Q3 last year. In that time frame, Microsoft began deploying WP7. WP7 is not the successor to WM6 or WEH. The line of upgrades will follow a different path.
As we all know, Windows Mobile is currently transitioning into Windows Embedded Handheld. Essentially, they are the same thing with a different name. Both use the CE5/6 core. Windows Phone 7 uses the CE 6 Revision 3 core in version 1. These systems are incompatible as we've been seeing. This breaks the upgrade chain for Windows Mobile. However, as I mentioned, WP7 is not the successor to WM6.
As WEH picks up steam in its course this year, it will eventually transition into another iteration of itself. In the second half of this year, WEH is supposedly going to begin using Windows Embedded Compact 7 (WEC7) as its core. This provided us with an alternative upgrade path from the dead end of WP7.
Windows Embedded Compact 7 began shipping at the end of last year to be used in slates and other consumer handheld devices. OEMs were encourages to add their own custom UIs. I don't have much information on WEC7 at the moment since it really hasn't hit any markets or news sources that I'm aware of.
Keeping that in mind, it is also of note that WP7 is set to begin using WEC7 instead of CE6 as its core in the next major release of the platform. I don't know if this will be in any way compatible with other WM6/WEH devices, but I would say it's unlikely.
It seems more likely that Microsoft is planning to separate their enterprise devices from their mass-market devices.
So to recap, Windows Mobile is set to be re-branded as Windows Embedded Handheld. Both using the CE5/6 core initially. Windows Embedded Handheld will eventually be updated to use the Windows Embedded Compact 7 core in late 2011. The platform will still be known as Windows Embedded Handheld at that point. This platform is supposedly aimed at enterprise level consumers.
Windows Phone 7 currently uses the CE6 R2 core. It is scheduled to be updated to the Windows Embedded Compact 7 core later this year as well. It will also retain its name as Windows Phone 7. This platform is supposedly aimed at the mass-market consumers.
Hopefully this clears up the upgrade path thing for most people. Stop worrying about Windows Mobile. The name is changing. It's not a big deal. It isn't a dead platform. (At least not yet, anyway. )