With the announcement of Windows Mobile 6 at the Barcelona Smartphone and despite a false start on 12th February the Mobile 6 SDK is finally available for download from .here
This is the full SDK and requires Visual Studio 2005 to be installed before you can install it. Microsoft has promised a standalone emulator release although they have not given a date for the release.
They also announced that there will be an update around 1st May which should see the 'final' version of the SDK being released.
If you think you will be working with Windows Mobile 6 then you might want to check out Nathan Winters March MMMUG event. This event has Jason Langridgehere.