As with the other example turn images off, or css off, or both off and the header text will still appear and make sense.
This demo originally used a span for the background image but has since been noted that the span isn't clickable in ie5 mac but an element such as an em is fine.
The drawbacks are the extra em required to produce the link in either of the cases specified above.
The background image in the em is placed absolutely so that sits on top of the text. If styles are disabled the image disappears and the header text is revealed. If images are disabled then the header text still shows through. If both images and css are turned off then the header text still appears as normal.
Other replacement methods that move the text off screen or use display;none etc will fail when images are off but css is still on. The above method doesn't suffer from this problem and users will always get something.
Screen readers will still read the header text as it is in the code in the normal place.
The main drawback is the extra non-sematic em. Unfortunately some compromises have to be made to make the effect work and after all nothing is perfect!
If you want a rollover image in the navigation then this can de done also.