It's a phone, for the following reasons:

1. You use it for browsing the internet
2. You use it for installing applications on
3. You use it for instant messaging

Same as the iPhone. Nobody ever said the iPhone wasn't a phone (not even taking into account the obvious: that it has "phone" in the name). So the N900 is a phone.

If you're in an emergency and someone shouts "does anyone here have a phone?", would you just go "nah, I only got this N900, so no phone"?

Do you carry another device with you that you use as a phone and then you use the N900 for other stuff? Why would you? If the N900 replaces your previous phone, then it's a phone.

And yeah, all the above is just my opinion. I agree that the line is blurry, but to me, the N900 is definitely a phone.