CSS table layout: why does table-row not accept a margin?

See the CSS 2.1 standard, section 17.5.3. When you use display:table-row, the height of the DIV is solely determined by the height of the table-cell elements in it. Thus, margin, padding, and height on those elements have no effect.

http://www.w3.org/TR/CSS2/tables.html

How's this for a work around (using an actual table)?

table {
    border-collapse: collapse;
}

tr.row {
    border-bottom: solid white 30px; /* change "white" to your background color */
}

It's not as dynamic, since you have to explicitly set the color of the border (unless there's a way around that too), but this is something I'm experimenting with on a project of my own.

Edit to include comments regarding transparent:

tr.row {
    border-bottom: 30px solid transparent;
}