The filament obeys Ohms law at any particular instant in time. It is true that the resistance increases as the filament heats up, but at any particular instant in time, V=IR. I don't necessarily agree with the way wikipedia labels the filament non-Ohmic. That's a little misleading. But then again, I've never heard the term "Ohmic" before either.

I think it is just a matter of definition. Some definitions of Ohm's law are confusing. Some books say it is an ohmic device if it follows ohms law and has a constant resistance. Still V=IR it is just that R changes.

Suppose we had some a battery with emf of V. There is a resistor with R. Then the power would be:
[tex]P=Vi=\frac{V^2}{R}[/tex]

The result of a changing resistance is a changing power. I mentioned power because for non-ohmic devices the power change could mess with circuitry. I mean we turn something on, it has one power and after it heats up it has another. For some situations that could mess stuff up and since the resistance changes infetismally with temperature it doens't have constant resistance. The changing R by some definitions means that is isn't an ohmic device.