oh, shut up...

Would it be handy sometimes send SMS's in portrait mode? HELL YES, specially when you can't hold the device with both hands.

Is it an outrageous proposition? no, it's NOT. You're all right about the N900 being a tablet not a phone, OK, but it has the ability to make phone calls and someone at Nokia thought it it would be USEFUL (and it makes sense) allow the phone funcionality to work in portrait mode.
So, if it's something software based, why is so hard to aply the same logic to the SMS's?.

It's something impossible for Nokia to do? No, it's not, it even WORKS with a Bug that allow you to use the phone on portrait, there are only a few changes needed.

Should this (portrait) be an option? YES, and those who hate it, disable it and keep using the device with both hands, and everybody happy.

The End.