You used to have to choose. Choose between an easy, but inflexible, px-based layout or a hard to control, but flexible, em-based layout.
Now with full-page zoom being implemented (as default!) in Internet Explorer 7, Firefox 3 and Opera 9*, it’s a different story. Full-page zooming means your easy px-based layout will be fully flexible. Even more flexible than most em-based layouts in fact, as images will scale along too.
So I’m asking myself, why bother with hard-to-keep-from-breaking em-based layouts?
[update] I don’t think liquid layouts are relevant anymore either.
[update 2] * And now Safari too! That’s all of the major browsers.
PS. This still leaves percentage-based layouts of course, but they maximise screen real estate. Which is very different from maximising readability (line-lengths and all that jazz). As far as I can see, %-based layouts are good for some web-apps (like gmail) but aren’t optimal for other uses.