One of the comments mentioned the drawback to using text with the negative indent - the selection border extending off the screen - which is why I suggest using pixel_trans.gif. Another possibility would be to have the text content be   (a non-breaking space), though it is possible that would have SEO issues.

Note the comment in the article that IE 6 & 7 have problems with links over foreground images. The standard Smart BG might be safest - test and see.