I have a long entry here about running your Windows machine as non-admin. In there I made a casual observation that it seems like people within Microsoft are finally committed to making running as non-admin possible within Windows. Since then, I found an article about Longhorn that actually says they are committed, and describes some of the things they are doing so that running as non-admin is actually possible.
All Windows developers should read this:
Security in Longhorn: Focus on Least Privilege