I have a problem with the margin-top in a nested div -- when I apply margin-top to the nested div, the margin is applied to the parent div inst
"Collapsing margins" is your problem. Here you could understand what is and why it's still alive: http://www.sitepoint.com/web-foundations/collapsing-margins/
I read across the web to look for a decent solution, and finally I found this article: http://www.seifi.org/css/understanding-taming-collapsing-margins-in-css.html
In short you have a bunch of methods to resolve your problem:
1) border in parent div (could be transparent)
2) padding in parent div
3) overflow: auto
4) float: left
You should follow the link because it explains in detail all the solutions.