A short Microsoft white paper describing Windows Mobile 5.0 user interface design guidelines.
“Following a summary of the new UI features in Windows Mobile 5.0, the article launches into the heart of the issue — suggested design rules and guidelines. The proper use of soft keys, dialog boxes, and notification bubbles is covered in detail. Finally, screen orientation and the problems of multiple resolutions are covered.”
UPDATE (20/1/11): New versions of Windows Mobile have been released since this post was first made. Check out the Windows Mobile 6.5 Guidelines, and the Windows Phone 7 UI Guidelines. Additionally, Microsoft published Smartphone User Interface Guidelines and PocketPC User Interface Guidelines.
Link: Designing Windows Mobile 5.0 application UIs (windowsfordevices.com)