I would say that the main difference between a mobile computer and a cellular phone is that on a computer you can compile/install any program/OS, while on a phone you wholly depend on your operator for updates and any functionality.

However, smart-phone blurs this distinction. I would say:
if you use device mostly for phone calls and SMS, it is a phone, maybe a smart one;
if you use device mostly for WiFi, it is an Internet Tablet, maybe a smart one;
if you use device mostly for GPS, it is a navigation unit, maybe a smart one;
if you use device mostly for camera, it is a camera;
if you use device mostly for calculator, it is a calculator;
if you use device mostly offline (office documents, texts, compilation of programs), it is a computer;
if you use device for everything at once, it is a multimedia device.

One point which can hardly be disputed: N900 is an all-purpose portable electronic computer (a cellular phone is an electronic computer with capabilities limited by singleness of purpose for the sake of endurance and performance). N900 is not just a phone or a tablet or a camera or a calculator; it's multitasking in all meanings of this word.