Google are now providing text links for Adsense / Adwords / Firefox referrals. Apparently added due to user feedback, it's a good feature -- sometimes a Big Shiny Button just doesn't suite your site design.
The text of the link is not configurable, but there are a number of options to choose from. The end result, looks like this.
I'm going to change some of my sites and see if this improves the click through rate. I rather imagine that it will.
Note: To see the text link adds in your Adsense setup, you must choose US English as your language (English UK will not work!).