I also centered in horizontally (that's what te width and the auto magind on body do.
To get this to work you need to create something (I used body) that's both 100% height of the viewport and has a minimal height of what you need. In addition you need to give that element position relative so that the abo#*$!ely positioned child uses it as a reference (and not the viewport -important should the viewport be too small)
The content itself is the absolutely positioned at halfway the height (top:50%) and the top is then moved back up by half the height (negative top margin).
That's basically all you need to get it working in FF, Safari, Opera, even IE (all latest versions).
Testing it in legacy IE versions left as an exercise to the reader (IE6 will have trouble with min-height, easiest is to throw in IE7.js )
With position:absolute you can easily break it when it's smaller than the viewport and scollbars aren't trivial to get. [they are in the example above.] Note that the body is made to be at least large enough to force the scrollbars from appearing and leave a large enough parent to position the content in even when it is smaller than the viewport.
This is probably a reason why so many think it's difficult to do. But the above should not break in a modern browser (it will in old IE versions I think, let us know if you can't figure out what to put in the conditional comments.)
There are ather ways to center vertically in CSS: position: absolute; top: 0; bottom: 0; margin-top: auto; margin-bottom: auto; height: 490px; will do the trick just as well in a standards compliant browser (I think some legacy IE versions will throw a fit over this though, making it harder to get it to work there)
For a position:static or position relative element all that is needed it those margins and a width. So either you go that path (like I did in the earlier example) bu horizontally centering a parent and work off of that), or you can also horizontally center an element that has position: absolute. It's much the same as for the vertical centering: either you do the left:50% ; margin-left: -Xpx; (where X is half the width), or you set both left and right to 0, do both margins as auto and set a width.
Note again that legacy IE versions won't be fond of setting both sides.