Actually this nothing to do with the operating system, it is perfectly agnostic to this, the item in question would be the phone application.
Logic often has little to do with requests for features, it is a desire to satisfy a functional solution. In your situation the use case would be useful to you.
The N900 is a 'talet like device' that is capable of making phone calls. Many laptops have the ability to utilise a sim card to make a cellular connection rather than plug in a usb cellular modem.
Just because it can make calls doesn't make the device specifically a phone any more than the introduction of fax protocols on computers made them a fax machine.
The N900 is a work in progress and was billed as suitable for developers and early adopters. If you didn't realise that meant it wasn't a device that was 'appliance ready' then that would explain your disappointment.
If portrait mode texting was such an important requirement how come you didn't realise this when you played with the device before you bought it? (Surely you didn't just buy it sight unseen?) You are also free to impliment your own solution too.