I’ve read through ‘hidden text guideline’ and I’m not convinced that it is aimed at penalizing image replacement techniques. It seems to be more of a warning against hiding text (excessive keywords) to manipulate Google’s search rankings. Shady SEO practices is what I see it aimed at.
All of the good image replacement methods passed the test with CSS and Images turned off. One or the other turned off, or both turned off together. There was nothing shady about the old image replacement methods, in fact they made sure everything was intact for all users and search engines. The text was left intact under the image for screen readers and search engines. The image was purely for decoration without any ill intentions.
Reading further and closely at the guideline you will see that they understand that not all hidden text is bad.
However, not all hidden text is considered deceptive. For example, if your site includes technologies that search engines have difficulty accessing, like JavaScript, images, or Flash files, using descriptive text for these items can improve the accessibility of your site. Remember that many human visitors using screen readers, mobile browsers, browsers without plug-ins, and slow connections will not be able to view that content either and will benefit from the descriptive text as well.
We used to be pretty well convinced that the <img> tag was reserved for images in the pages actual content. Whereas background images are used for decoration, as with the image replacement methods.
The good methods did not position the text off screen or use font-size:0
That would not pass the Images/Off test.
You can test your site’s accessibility by turning off JavaScript, Flash, and images in your browser, or by using a text-only browser such as Lynx.
Lower down in the page you will find the Irrelevant keywords link. I think it goes hand in hand with the hidden text guide.