Although I don't particularly like doing it with an outdent, I just tried it in DE and it does work ok. Using ems does make this scale correctly with font size.
Did you look at the epub I posted? What I was trying to do there was this:
p { margin-left: 4em; }
span.pnum { float:left; width:4em; }
<div>
<span class="pnum">1.1</span>
<p>Now is the time for all good men...</p>
</div>
This works just fine in FF, but DE inserts a newline after the span. I assume that it is a bug in DE, unless I'm missing something obvious.