I guess the answer is never?
Came across this, from 2008, when I was recently re-jigging my site and wondering whether there was actually any reason anymore to swop everything over to EMs from the pixels I'd inherited with the theme - which I'd been intending to do since day 1, but you know: laziness and all - seeing as how modern browsers such as Chrome will zoom all the content anyway, regardless of whether you've used pixels or EMs. And after a great deal of reading I have come to the conclusion: It just doesn't matter anymore.
CSS pixels and EMs aren't that different anyway. It is possible to do fluid layouts with both. And although Chrome will only do full page zoom, not text zoom, other major browsers (I don't consider IE to be 'major') that do offer text zoom will zoom pixels.
However, now I've made the E(m)ffort I'm not changing back (you know: laziness and all).