There's been quite a few posts in the last few years about using CSS Image Replacement techniques, this has made some wary as they use words like display: none; visibility: hidden; etc. And the possibility has always been there for the technique to be abused. I know tedster has mentioned a few times that opinion on its usage even within Google has been split at times.

My opinion has always been that SE's could not simply penalise everyone using it at is a legitimate accessibility design technique, i.e. you can provide a text alternative for a graphic should users have CSS and/or Images turned off. If you can provide it for a user you can provide it for a bot.

Yahoo have been open about it and recommend using CSS Sprites as one way to speed up your pages, Google now seem to be doing the same.

I don't know if I missed this being reported here but Google are now using Image Replacement on their SERPs and it's a CSS Sprite [google.co.uk].

T.V Raman writes on the Google Blog: CSS sprites and navigation links [googlewebmastercentral.blogspot.com]

Having meaningful text to go with navigational links is equally important for Googlebot as well as users who cannot perceive the meaning of an image. While designing the look and feel of navigational links on your site, you may have chosen to go with images that function as links, e.g., by placing <img> tags within <a> elements. That design enables you to place the descriptive text as an alt attribute on the <img> tag.

But what if you've switched to using CSS sprites to optimize page loading? It's still possible to include that all-important descriptive text when applying CSS sprites; for a possible solution, see how the Google logo and the various nav-links at the bottom of the Google Results page are coded. In brief, we placed the descriptive text right under the CSS-sprited image

.