You must add portrait support for most areas of the OS. I was sure that this was going to be my next "phone" (mobile computer) but what is the point of device which is pocketable which cannot be used on the go. I may as well put it in a bag. Think about it, the majority of the time you see someone with a touchscreen device they are using it in portrait.
Examples:
1)Walking to work want to check my emails casually - single handed, so that I can carry my briefcase.

2) Check RSS feeds out and about.

3) Check the time single headedly.

4) Using Ovi Maps while walking in an unknown place (If I try to do this with 2 hands I will probably get hit by a bus - by adding portrait support you will actually be saving lives!

I want to be able to multitask on the phone (mobile computer) but also in real life. Remember this: You were not criticised for the lack of portrait support at Nokia World because everyone was simply demoing the n900 not trying to use it in "real world situations". This will be different for the consumer.

The lack of portrait support is a total dealbreaker sadly.

Suggestions:

Menus: These can easily be changed to portrait (I'm not a software developer so I making assumptions here).

Dashboard: Arrange all the windows the same way only rotated.

Browser: Like other mobile browsers address bar at bottom, with onscreen portrait keyboard etc...

Camera: keep the same, obviously.

Messaging: Use onscreen portrait keyboard.

Start with the key apps for mobility before moving onto others. By doing this, people will not be so frustrated.

In conclusion, it is sad to see that you have forgotten that you are designing a mobile computer NOT a computer. Thus, it is crucial that you delicately balance your two objectives to avoid total failure.

Computer: Full marks
Mobile: 70%