Turns out that text indexing (-9999px), negative margins and positioning are bad for google! http://maileohye.com/html-text-indent-not-messing-up-your-rankings/
What search-friendly technique would you use to hide the text when using sprites?
Clipping the text may be a better solution, although I'm so far unable to really turn up anything definitive on Google's view of this. But see this Yahoo post for more info...
Reading the comments to that post there doesn't seem to be an answer. Assumptions are dangerous, but I have to assume that Google looks the length of the text and weights it accordingly. For example if the hidden text is "My Road Trip" it takes that into account, where as "My Easy Road Trip about Making Money Free and Easy" and even longer sections of text being hidden starting to more and more like spam and getting discounted.
I personally don't worry about it when using sprites for menus and such. I would not however do it for larger relevant blocks of text, a pulled quote for example that I wanted to be in a fancy font... for that I'd use webfonts.