I don't think it's very meaningful to determine computer/not computer based on individual technologies. You can't even get everyone to agree on the best window manager or text editor. I'd propose we call devices that can run existing desktop apps mobile computers and devices that can't smartphones. That's a nice and clean line, and shouldn't cause too much controversy.

In practice that means it needs to use X11, a normal filesystem layout, and satisfy the usual dependencies. The N900 qualifies, the others do not. A Linux kernel alone does not make Android or WebOS a "Linux" operating system distribution, and the iPhone's approach is a complete joke. Nokia is the only one that gets it.