I have some funny effect:

• (from Desktop) physically rotate N900 to Portrait
• Startup a program (like contacts, starts up and rotates to portrait)
• Work with contacts in portrait mode
• Close contacts and leave phone in portrait position
• (Rotates to landscape and shows desktop)
• Select status menu

The status menu should show up in landscape mode. But I had the effect that it rotates to portrait and when leaving the menu everything rotates to landscape desktop again. I had this effect twice and it is not 100% reproducable. I think that this is somewhere between closing the program in portrait and going back to landscape Desktop.

Update: You can only fix the rotating by starting a program in landscape afterwards and leave it.