You could probably implement this entirely in CSS. It's a one-layer menu, so using display:none combined with a hover element could hide and show the stuff.

The problem is figuring out just what you're going to put into that menu. Nielsen's point is that these aren't single-list menus: they're complex and serve multiple purposes at once. So figure out what your menu's going to do before you do anything.

Could you add the 0.5 sec. delay and cover the diagonal issue with just CSS?

I went to Nielsen's first example (http://foodnetwork.com) with NoScript enabled (meaning it degraded to CSS-only solution) and it worked quite well.

It was actually quite hard to get diagonal issue to be the problem, you must follow rather unnatural path to get out of the active zone.

And 0.5s delay was detrimental, site felt snappier without it.

I'd think you'd need javascript/flash for that.

You'd have issues with our favourite browser trying to do it in CSS exclusively. I'd go for javascript for the next few years at least.

Would it need to be absolutely positioned on the page? If so, how do you front-end guys handle this cross-platform?

I've never done this before: consider this entirely speculation. However, I'd probably use relative positioning. This is speculation - I haven't opened Coda to try this - but perhaps make each top button a div that reveals its child content when it's being hovered over. That way, there's no confusion over where the drop-down menu's going to appear. It might be a little tricky fitting the things that appear under the drop-down menu, but since relative positioning allows for overlaps, I'd say that this would be a fairly simple issue. Drop-downs in CSS aren't particularly difficult: they just get more complex the more children you add.

You could use relative positioning, and if need be, serve an alternate stylesheet through conditional comments.

I couldn't find one when I implemented drop downs a few months ago, but the hover intent plugin was very useful for rolling my own.