By default, the N900 is basically set in landscape mode. To force applications to portrait mode, do the following.
If you’ve not done so yet, add and enable all repositories.
Install the Community Seamless Software Updates
Install a small package called CSSU Features. You can either search for it in the app manager or use the terminal
root (assuming you have Rootsh installed)
apt-get install cssufeatures
Go to the newly installed Cssu features, scroll down to Forced App auto rotation and choose all apps.
Click on the update button.
From now, all applications will be forced into portrait mode if the phone is so rotated.
Please know that the CSSU is still under development and you intall it in the knowledge that you might have to reflash your device should you brick it in the process.
I’ll not be responsible for any nuclear war started as a result of a bricked or broken device…:-).