Well. i did some research and this seems to be the rule:

In FF or safari if you float an element in a parent which does not have float or position set the children will float outside the parent.

In IE 7 floated child elements stay inside their parent even if that parent has no float or position

In IE 6 it is similar to FF/Safari except setting a width on the parent works as well as setting a float or position.

Does anyone know what the standards are?