mihomes - 7:20 pm on May 8, 2012 (gmt 0)
My advice would be to use a conventional <a><img></a> rollover if you can't style the text satisfactorily with CSS.
... I've thought about this as well. I could keep the title attribute and/or use an alt attribute as the image would be there. Problem is no anchor text at all (although adding the alt tag would help this a little) and I would no longer be able to use sprites to optimize these buttons unless I defined a transparent image in the html and used css to show the actual button backgrounds/rollovers.
I've been doing this for years and is completely false from what I can tell.
I see there are already two conflicting responses. I have to agree with you though based on 'logic'. You can use text-indent all you want to hide text, but to a search engine they are always going to see that text. In other words, if you are trying to hide keyword stuffing or what have you... you are only doing this to the user and not the search engine...which it would still penalize you if need be. I never thought about the manual review trigger though which makes sense as well.
I am going to ponder over my options a little more, but I think I will end up with the hidden anchor text and title attribute only where it would help (say mutiple buy buttons in a table on an order page).