BH, It's collapsing margins [webmasterworld.com]
First off, Firefox is the correct rendering ;)
The margin of the h2 is collapsing/combining with the margin of the #news div because there is nothing (speaking in Box Model terms) to stop the margins adjoining. When you add the border (padding would do it too) it puts a "barrier" between the margins of the two boxes and stops the margins adjoining, therefore stopping them collapsing.
You would normally see the same effect (as FF/OP) in IE too. IE renders collapsing margins properly most of the time when a specific top margin is used on the elements involved, which you have in this case.. however our old friend hasLayout (triggered by the width on the news div) comes into play and causes a double whammy making IE not collapse the margins in either case. :o
I take it it is the IE interpretation you want, in which case if you do not want to use a border use 1px top padding instead.
edit reason: added reference link - or try this search [google.com]
[edited by: SuzyUK at 3:59 pm (utc) on Dec. 5, 2007]