Introduction to Apple Software Design Guidelines
Lot's of good advice here (from a "birds-eye-perspective-in-an-ideal-world-with-no-legacy-code", though) in the Introduction to Apple Software Design Guidelines. There's also a PDF version. (via Ranchero.com).
See also:
- Apple Human Interface Guidelines (PDF)
- Windows Guidelines for User Interface Developers and Designers
- Windows XP Visual Guidelines for Applications