Am I seeing things right that if one wants a 15" WUXGA screen, one has to get a W500 laptop as it wont be in a T500?

As of now - that is the case.

So is the W500 to the T500 the same as the T60p to the T60, that is, same thing with highres screen and slightly upped specs, or are there substantial differences?

That seems to be the case. A different graphics card and the availability of the WUXGA screen.

The main difference is the same as what it has always been with the p-series - ISV certification for OpenGL applications.

I just ordered one with the WSXGA+ screen. It was only about $50 more than a similarly configured T-series and I think they hold their value better. p-series machines always have.