you don’t need a third party service some tiny JS snippet will do the job:

if (screen.width>=1024 && screen.height>=768) {
window.location="highres.html";
} else {
window.location="lowres.html";
}

of just include the right CSS style sheet after the resolution is detected.

But anyway you have to develop multiple websites to serve all the resolutions.

2 other factors that I think are important are:

screen real estate usage, and
line-length for readability

I often hear people quoting resolution stats, but what those numbers don’t tell us is how much of the screen the browser is actually displaying. Some users don’t open the browser to its full size, others may keep their histories, delicious boomarks or other content open in a side window (thus narrowing the viewport), while others, like me, have extra toolbars installed that significantly reduce the vertical space. Thus even a user with a big screen may not be seeing the whole screen.

Line-length is something that becomes a challenge with liquid layouts. We know shorter line lengths are easier to read, but they are far easier to control in a fixed environment. With a liquid layout one can fix the width of columns to prevent text from stretching out too wide, but that often looks weird as well. Yet if we let the text stretch it can become very hard to read. And of course we all know the challenges of designing something that looks good at varying sizes.

I don’t know that the debate will ever end, but I think as long as we keep reminding people that it is not only a question of screen size, we’re on the right track.