Microsoft is officially hitting the reset button on their Windows Mobile series. They officially confirmed that all apps on previous versions of Windows Mobile will be incompatible with Windows Mobile 7. They also confirmed that most (I think all) current Windows Phones will not be upgradeable to Windows Mobile 7. Not even the HTC HD2.
New developer tools will also be available for the new OS. These include Silverlight and XNA developer tools. Microsoft is also being very strict on their hardware specs. The main specs that all new Windows Mobile 7 Phones will need to have a minimum of 3 front buttons: Back Button, Windows Logo Button and Search button.
So far the only phone running the Windows Mobile 7 OS is the LG prototype here:
Microsoft will probably unveil more at their MIX 10 this month. Check back for more details